Fastest way to improve aerobic base without overtraining?

Biking requires considerably more time training.

There isn't an easy way to reduce your ideal training time unless you find a way around the time constraints that limit your running.

You can run a great marathon on 60 miles a week, it just depends on how you structure it. If you did 10, 12 and 20 mile runs then you only have to jog <5 miles the other days.

if I were you I would in tease your mileage by adding volume to your hard workouts/long runs first. Make them tough days, and then use the other days to really really recover. I think one of the main things that has kept me healthy the past 3 years is running slow on easy days, taking days off when I need to.

in my opinion there isn’t a huge reason to spend tons of time cross training. I think a lot of cross training would just be a hinderance to your running recovery, unless you specifically use it as recovery

Why would it require more time? If I just go by HR, wouldn’t an easy 75 mins on the bike at 140bpm similar to 75 mins jogging, minus the wear and tear?

The risk of overtraining on a bike is much-less.

When I was unable to run for 2 months, due to an achilles injury, I rode hard 5 out of 7 days. When I say hard, I mean my HR was above Lactate Threshold for at least 20 mins of volume each time.

I still ride hard 2 days a week on top of my 2 hard running workouts per week.

No, because 75 minutes on a bike is a lot easier than 75 minutes running. And if you can bike for 75 minutes why not just run for 75?

Thanks. Yes, that is the intention if I were to do it. My question is if biking can make it safer/faster to increase mileage by substituting regular runs or the second runs of the day with a cycle. I say “faster” because I imagine that trying to increase my aerobic base only by running will bring a higher risk of overtraining or injury from the pounding of the roads and thus cause me to add mileage slowly. Biking (or swimming if I had easier access) seems like an easy and safe way to build up general aerobic fitness when you’re trying to rush fitness.

The idea is to avoid potential injuries that could come with trying to rapidly increase mileage. Regarding it being easier — sure, it’s not employing all running-related muscles, but that’s why I suggest going by HR. In my experience, it takes a lot more effort to get my HR up on a bike than a run. The goal would be an aerobic HR zone, minus the injury risks.

Nope. Biking requires considerably more training time to get the desired effect. 75 minutes of running is waaaay harder than 75 minutes of biking because your weight is supported by the saddle and the handlebars.

On a bike your legs can keep turning at 140bpm for hour after hour.

So don't try to increase mileage rapidly.

The reason you struggle at biking is because you don't have the experience to pedal efficiently, but aquiring that technical improvement on a bike won't make you a better runner.

Both activities are skills, that's the hardest thing to develop. Look at Kipchoge's technical skill as he approaches the finish of a marathon. What do you see?

For sure the caloric expenditure would be greater running than biking, BUT 75 minutes of biking at let's say 65-70% of max heart rate (easy aerobic) would give you the same cardiovascular workout as 75 minutes of running at the same effort.

Keep in mind, however, that max heart rate is lower on the bike (usually about 10% lower),
so you'd need to adjust biking heart rate to the desired percentage of biking max heart rate.

Finally, if you have access to an elliptical trainer, that'd also be a nice way to add aerobic volume while also doing weight bearing exercise, but without impact. There are some very fast US collegiate ladies with history of impact injuries that do a lot of their easy/medium aerobic work on elliptical and have had great racing results.

My heart rates are just as high on a bike. Maybe you lack biking fitness. Also, you can't compare the "cardiovascular workout" running versus biking by time, that doesn't make any sense.

Why try to reinvent the wheel? If you have the time, you can build up to huge training loads in running if you do it gradually and carefully.

But the OP is time constrained. I don't have a solution.

Aerobic base is MORE about training your muscles than it is about training your cardiovascular system. Especially so for marathons. You have to create specific aerobic adaptations in as many muscle fibers as possible. For this, you have to tire your easily available fibers and recruit those that are normally not recruited (long runs and high mileage does that). You have to train maximum number of muscle fibers to glycogen deficit so that in response more fat and glycogen is stored in those fibers and greater mitochondrial mass is achieved. Cross training on the bike you will not adapt optimally for running, but it will help. Somewhat.

The point is really my first sentence: aerobic work is aerobic work.

As for how it helps OP? Touché.

I certainly agree with you that running is the best training for running, and that specificity of training is important. Specifically training the running muscle fibers is the way to go if you can do so without injury.

Better off running for 30mins than riding the bike for 75. Who ‘overtrains’ in base anyway? Just run slow, maybe one tempo run a week.

True, but don't forget, running is a skill, you have to train your brain to use those muscle motor units in better and better sequences to maximize efficiency and endurance.

Yes, 30 minutes running is worth a lot more than 75 minutes biking. Also, biking doesn't strengthen the feet, ankles and heels the way that running does. If anything it makes them weaker.