Will cross country kill my speed?

Yes, exactly.

The shorter the event, the bigger difference additional seconds make in terms of the physiological demands. This is obvious when we compare events--there's a reason event distances increase roughly exponentially (200, 400, 800...) instead of linearly.

The additional 10-15 seconds that separate similarly competitive boys and girls at the HS level are 20-30% of the race, which makes it a much more aerobic event. Even talking in terms of percentages, this is much more significant than a 20-30% difference would be in 10k, where the event is 90+ percent aerobic whether you're running sub 30 or over 40.

Absolutely. With 58 speed? Yes yes yes.

Go out in 67, see what you can do. I'm guessing you'll be under 2:20 before you know it.

Clyde Hart is the paragon of strength- and endurance-based 400 training, and it's worth noting that the longest runs he recommends in the fall are usually 3-5 miles, depending on the athlete. Not incompatible with cross country, but most cross country runners would run farther than that. With that in mind, I think a smart coach would make significant modifications for any XC runner who was a 400 specialist.

Ruin it? No...and whoever said that is a jackass.

What might ruin it is listening to half the advice listed here.

Like some have said, do some strides and hard hill sprints (10-15s) a couple times a week. You'll be fine come track with the added bonus of an XC base.

long story to be told here, but for a real quicky, 200/400 is on the longer side of sprint and the hills will build you some strength for the speed. it's your mileage over the course of a week without some speed training that will kill you, not cross country itself. if you can pull off sub 30 mpw and the coach is understanding of your needs, you likely won't be affected

with that said, here is a story from my high school. there was a 400m (0:48s) guy who wanted to stay in shape for track so he asked the cross country coach if he could do low level stuff and the coach agreed. it turned out that he was exceptionally good. ran sub 19 in his first 5k and moved it down to sub 17 by the end of the season. then it was track season again and he ran 1:55 in his 800m debut. by the end of senior year in high school, he was a 1:48 guy so it turned out that he was running in the wrong events this whole time.

moral of the story: cross country COULD hurt your performance in the 200/400, but you might discover that you have more potential in middle distance or long distance than in sprint. perhaps try your best in one season and find out where you stand

My daughter ran cross country and took 8th at state in the 100Hurdles! I tried to get her to do the 400 and 300IH but she ran the 100HH, Long Jumped, and High Jumped! She wasn't bad at cross country either.

So?? Why does her national rank matter? Her question is still legitimate and she is talented. You don't have to be super talented to worry about losing some natural speed or ability.

XC builds strength (endurance/stamina) Track & Field builds speed. What is Indoor track? We never really had it, especially in high school.

did this guy try any 800s or 1600s?

No, XC won't hurt your track sprint speed. Doing the endurance-based training may,
Temporarily, slightly change the metabolic characteristics of your fast twitch fibers to more aerobic in nature, but if your focus gets back to more sprint-based training for indoor track, your natural explosiveness will return. Also, as others here have said, it'd be a good idea if during XC you do some weekly very short sprints on flat or hills to help stay in touch with explosive power (these should be done for all XC folks anyway).

As an example of one, there was a girl here in Ohio who won state titles in XC (18:04) and in the 100m (14.29) hurdles. She also ran 1:00.3 sec for 400 hurdles, and pole vaulted
10 feet even.

She went on to Stanford, where she ran 2:00 for 800 and 20:24 for 6k XC (sub 5:30/mile pace).

So, I'd say you're okay to do XC. Just have fun and don't worry about it!!

How do you know it's a girl?

Funny you should mention Monique Henderson. I was around her a little bit when she was in high school and she did indeed run 18 and change for 5k, she also had a 30 meter fly time of 3.08 seconds which equates to an 11.10, 100 meter dash time, but she only managed to run 11.61 in the 100 meters, along with 23.16 in the 200 and 50.74 in the 400 in the year 2000.

The first 30 meters of a sprint is all strength and power and training aerobically diminishes that. You only build strength and power by training anaerobically.

But the longer the race the less important the first 30 meters is. Training aerobically will also not give you more endurance in an anaerobic event but it will give you more tolerance for the lactate acid pain.

Using Monique Henderson as an example is kind of unfair especially since the original question was posed by a girl who is running 26's and 58's. My answer would be, why are you not focusing on cross country and distance running in track as your main sport as you seem ideally suited to excel in those events?

No it won't but I will say when I have a sprinter doing XC the volume is lower and so we do a little bit more leg turner stuff. So they maybe in the 30-40mpw groups overall. But depends on the kid and their goals. Are they just staying in shape and active or do they want to run fast and maybe make varsity one day. I know 16min to sub 16min guys that ran sub-50 in track.

UPDATE: Cross country was not the way to go at all. Got way slower than last year, I'm mentally burnt out, and I got bad tendonitis in my hip from the overuse. Would not recommend. Crossing my fingers that I can get back what I lost and maybe even improve a little from last year.