I know a guy who ran well over 26 minutes for his first 5k in high school and recently was top 5 in his XC region at a D2 school. The best guy on my XC team my freshman year of college was the worst runner on his middle school team and wasn’t good until his Junior and Senior year of high school, he ended up running well under 15 minutes in the 5k. It’s not where you start from, it’s how quickly you progress.
You can certainly get ready for indoor track, figure out what events you want to do, contact the track coach at your school and tell him/her you are interested. They should let you on the team, but just explain to them you play football and won’t be doing XC. If you want to be as successful as possible as a runner, you’ll need to drop football and commit to running year round. If you want to be a distance runner (800, 1600 or 3200) the coach might require you to run XC, this is a decision you’ll have to make depending on how much you want to run. The training benefit of running year round will give you a serious leg up on anyone else that doesn’t run year round.
As for training, you can join the XC team, drop football and follow what the coach says. If you still want to play football, you’ll need to have a modified training schedule, if the coach gives you any training to do, do that. If the coach can’t give you anything, I would recommend doing just easy running. Don’t worry about the pace, you want to be running at a pace you could hold a conversation at. If your best mile is 7:58, that pace would most likely be no faster than 10:00 per mile, but as you run, that pace will get faster because you’ll get into shape. A general rule of thumb is your heart rate should never be above 160 bpm, and to test this, you take a quick break in the middle of your run, get the pulse on your neck, and count how many pulses there are in 6 seconds, and then multiply by ten. If the number you get is 170 or more, slow down. You shouldn’t feel tired at the end of an easy run, the point is to develop your aerobic system to be ready to handle harder sessions and races in the coming track season, not to make you as fast as possible right away. I would recommend a maximum of 30 minutes a day of easy running for at least the first 2 weeks, that should get you into decent shape. Then you can add in a longer run once a week (1 hour max), and make a couple of your runs in the 40-45 minute range. Make the increase gradual though, and if 30 minutes is too hard for you initially, just do whatever you can and work your way up so 30 minutes a day of easy running is not difficult. If you make any changes to your training, I would recommend doing the new training for 2 weeks. Your body will need to adjust to the training load, and too much too fast will most likely injure you. A rule of thumb with increasing mileage is to not increase by more than 10% week to week, so say you ran 20 miles in a week, the next week shouldn’t be more than 22. Don’t be a slave to that rule though, if you go a mile or two over it’s not a huge deal, it’s just to prevent somebody jumping from 20 miles to 40 miles.
If you are playing football, you should be getting plenty of strength training in. If the track coach gives you stuff to do, just add that into your routine. Strength training for distance runners isn’t overly difficult and is mostly just body weight exercises. One thing that would be helpful is to add in hill sprints 1-2 times per week. So pick a day or 2 and cut your run a few minutes short and start off with 5 or so 10 second hill sprints up a steep hill. Give yourself as much recovery time as needed to go all out for each rep, the goal of this is to develop strength in your legs. Once 5 becomes easy, you can increase the reps, but always keep the recovery long enough to where you aren’t struggling up the hill.
Good luck my dude.