Fastest way to improve aerobic base without overtraining?

I'd say thats there's almost no limit to the amount of low/moderate exercise you can do on a bike or in the pool. Its just the case in running that unless you're exceptionally well conditioned (or have fuelled dreadfully) your muscles will give up before you run out of gas.

75 mins on a bike is the same as 75 mins running if the HR is the same in terms of CV response. But nope, it won't be completely applicable to running as there's more to it than how much oxygen your lungs / heart can convert to something useful for your muscles.

The best way to improve your aerobic base is to run as many comfortably paced miles as you can. But if you're at a point where you feel like doing more of that could be detrimental then sure, some of cross training isn't going to do any harm and may do some good, though not as much good as more running will. The two things here I'll suggest is either cross country skiing or simple walking, not race walking though that would be good too.. Both offer the muscle movements most like running. Before you dismiss walking I'll say that if you look at the training of long distance runners in the late 19th and early 20th Centuries you'll see that many of them did a considerable amount of walking as part of their training.

This is going to sound weird on a running board especially with serious runners, but I will build on what HRE said. Two years ago, I built back my fitness which led to an eventual marathon PR by walking an extreme amount. I walked 500 miles in one month which helped as I transitioned back to running.

But further, late last year I had an injury that sidelined me off my feet for a while. In that time, I cycled some but also ended up resting. I needed to rebuild as quickly as possible to be ready for Boston. I made my own plan which consisted of a lot of walking, some cycling (based on my own conversion) and then a gradual build of running miles. At first, I could only run every other day. I was surprised when a month ago I had a great improvement in my cardiovascular fitness (as measured through HR) and my speed is almost 100% back. I got up to high 50s/60s running miles/wk and around 18 walking miles/wk. My legs were certainly tired. As far as how it goes, I'll report back but I'm feeling confident.

Cross training can help 100%. You need to go hard on the bike. I know International marathon runners who've done mostly cycling for some blocks with injury.

Plenty of injuries are loading intensity and distance at same time. If the marathon is September you can put in a sensible base phase now and add bikes in immediately as it won't hinder workouts as long as you stick to base. That will definitely speed up the process for the fitness you can bring to a marathon block as long as you start now. Just phase out the bikes as the running load increases ....easy.

Inclined treadmill walking could do the trick ...

The quickest way to build aerobic fitness in the least time is to do 2x5 minutes of 20s hard, 10s easy, twice a week. That's a total of 20 minutes a week. You'll have to warmup and cool down. This built more speed and endurance than an easy 45 minutes a day in tests.

Yeah, twenty minutes of running a week should have you winning marathons.

Then why aren't triathletes the best runners in the world?

Van der Poel has admitted that he is a lousy runner (something like 36 minute 10k).

XC skiing is a better crossfit to running than biking. I recall a Norwegian Didrick Toenseth lost a couple seconds only to one of the Ingebrigtsens at the Oslo XC running 2-3 years ago. And if it were a real trail he would outperform.

I swim 3 days a week. I started cross training to keep miles off my legs and prevent nagging injuries but I do notice I can swim pretty intensely and recover much quicker than I do from running. TBH, I have no idea whether this improves my running form other than maybe keeping some weight off but it seems to keep me fit and healthy. Granted I just turned 40 yesterday and have no aspirations of beating my high school PRs. Good luck!

Intuitively, the idea that a given amount of time at a given HR should equally improve cardiovascular fitness/aerobic base, regardless of the activity, seems to make sense. But this discussion makes it all the more clear that cardiovascular fitness/aerobic base is *not* just based on conditioning the heart as a muscle (analogous to doing bicep curls!). I love swimming, and it is wonderful exercise but, unless you need to lose bodyfat or are are a completely unconditioned non-athlete, watch how little this helps running, even if you do it at a high HR. (Makes me think of my old masters group, in which many male swimmers in the fast lane were incredibly fast, yet many had big bellies and could hardly run at all.) As others have posted, there are many physiological adaptations to training that contribute to running aerobic base, and all cross fitness activities are not "equal" in contributing to it. Cross-country skiing is an example of one that translates very well (see Ingrid Kristiansen's training); cycling some, but less so; and swimming hardly at all. Interestingly, it is not even equal in both directions - I remember there were studies that found that cycling (and especially doing running-like standing sprint intervals on the bike) translated pretty well to running fitness, but running translated *much* less to cycling fitness. As others have pointed out, none of the alternatives to running will be as time efficient for you. If you must cross-train in order to reduce risk of running injuries, and although much less time-efficient, cycling indoors on a trainer, elliptical machine, water running, cross country skiing or crosscountry ski machine, and perhaps even high volume of fast walking (especially up hills) could help some to supplement the running. But prioritize running as much as you can w/out getting injured.

I am coming off of injury and did my first running based workout today. Adapted it from the style of Allie O's treadmill stuff, 5 minute reps at 10% grade at 5-5.5 mph. Super easy on the legs and got my HR/lungs working. Will be keeping this in the rotation for sure