Sprinter shifting to distance

I'm not sure what your training looked like during XC, but that would probably be a good place to start if you're really looking to get into distance. The XC/sprinting combo thing was a really good idea, I had teammates who did it and it helped them a lot during track season, and if you enjoyed it then it was totally worth it.

If you've been focusing more on sprinting and speedwork then you're probably going to have more of an issue with endurance. The important thing is to build up your mileage really slowly because if you do too much too soon you'll get hurt. What does your plan look like right now?

Thanks for getting back to me, I just rebooted my early xc routine. Our coach had us run by minutes instead of miles. Its 3 runs that go from about 20-30 minutes a week, and between those short runs there's 2 longer runs, starting at 40 minutes and increasing to 80 after 2 months, and a Saturday "long" run, starting at 6 miles and increasing to 13 after 2 months, adding a mile a week. It's loosely based on some lydiard principles is what I've been told

Your 400 time is a big drop off for someone who can run 12.5 and your times drop off even further as the distance increases. I think you should continue exploring your sprint development but maybe you've only done very little distance work. Still, that 12.5 is by far your best performance and the 25.xx is good considering. The XC training you've done may have taken away from your sprint performance (by converting some muscle fibers to slow twitch), why not try doing some plyo work and sprint drills starting in Winter to prepare for track.

Yes, people have intermediate fiber types which can change between slow twitch and fast twitch behaviour based on training.

If you are in it for enjoyment and fitness then run whatever distance you enjoy. The great thing about running LD is how it strengthens the heart and lungs.

My favorite part about running is the noticeable improvement throughout the process, so I'd like to get into road 5-10ks and maybe work up to a marathon or a half, and I'd like to do these productively.

That seems like a good plan, the only thing I'd suggest is that if you haven't run much lately you might want to start with just running 3-4 days a week to get back into it.

In terms of workouts, tempo/fartlek stuff would help a lot with endurance. I don't know what other teams do so if you've never done a tempo workout before, it's basically alternating between running at a "comfortable hard" pace (you shouldn't feel able to hold a conversation, but you shouldn't be going all-out either) and jogging. You can do it for time (i.e. 4 x 3minute tempo with 3 minutes of jogging between each tempo) or for distance (4 x 800m with 800m of jogging between each tempo). Sometimes my coach also had us add tempo to the end of a long run (so we would do our long run normally but tempo the last mile), but usually that wasn't until later in the season.

Also, you probably already know this, but don't neglect easy days or days off. If you try to go hard every single day you'll burn out and get hurt. You should also consider taking a day completely off from running at least once every two weeks (my team does it once a week but that's just my experience).