Mileage or time for a beginning 12-14 year old athlete

There’s no one size fits all answer here. My rule of thumb at that age is that you can never do too little but you can definitely do too much. You’re likely to get some kids who can’t even run one mile without walking. Meet each kid with where they are at. FWIW, Bowerman Track Club’s youth program’s athletic.net profile has the following:

Age 8-10: 3 or 4 days a week of running, Longest run not to exceed 6 miles, Maximum 15mpw
Age 10-12: 4 days a week of running, longest run not to exceed 8 miles, Maximum 20mpw
Age 12-14: 5 days a week of running, Longest run not to exceed 10miles, Maximum 30mpw

Those are ceilings, not targets or floors. Definitely don’t exceed them and for most kids don’t get even close to them.

Tell those creeps to do a real sport. You suck, “coach”. Haha 🖕

I like time when there's laps available because everyone finishes at the same time. Sucks to be those kids still running while the faster kids are done. This year's slow kid can be next year's fast kid; you don't want to lose them. Plus those faster kids can get some extra distance in. In the end, you're building a team, so work them as a team as much as possible. Good luck!

I would have them start running about 20- 30 min a couple times a week during the summer. Encourage them to run in groups or in pairs. Have them keep a log and slowly increase their weekly times. Try and have them focus on running by feel. Maybe have a fun run once a week and check in with each athlete to see how they are improving. Maybe start adding strides to some of their runs or whatever you think using the run you feel.
August is usually when teams start to hold organized practices. You can find so much online geared towards younger runners.

Good luck.

Have them run/walk an hour a day

Week 1 - Jog 30 minutes Walk 30 minutes

Week 2 - Jog 35 Walk 25

Week 3 - Jog 40 Walk 20

… you get the idea until they’re jogging an hour 5-6 days a week.

Preach that all you need to be a good runner is an hour a day.

Wait till September introduce tempo workouts.

Dont worry about pace or distance. Tell them to run at pace they could still hold a conversation.

The walking is very important. It physically and mentally prepares them for more running but is low impact.

Play ultimate frisbee a few days a week to get some speed in. It’s fun too!

Get them in the habit of core work like planks and bridges. These exercises will prevent injuries and only take a few minutes.

Tell them not to wear Nikes and get a decent pair of Brooks or ASICS. This will also avoid injuries.

Have Fun!!!

A serious advice would be not overdo the aerob part of their training. At 12-14 their hearts aren't ready for that. I would have emphasized on developing their natural sprint speed.Good luck!