Anglo - Following a Wendler article, I started sprinting hills... I have not missed a workout of doing every other day for about 5 months now, NOT A DAY. I have gone from 230 to 200 lbs and my pull ups went from 1 to 10. I haven't lost any strength, have lost a boatload of fat, and my cardio/conditioning is way up. I am 30 and haven't played many organized sports since college.
I was called up by a few buddies to go play full court basketball (4 of the 10 guys in the gym played college ball in the last year!), and I was in as good a shape as anyone on the court (including a few guards built to run and jump like gazelles). Anyways, it is getting me toward my goals and I love it... it's more of a love/hate relationship (the gift and the curse), but it is working for me.
I also err on the side of slower speed w/ less time between reps for the cardio benefit and b/c it's a beating. The first few weeks I did this, I hit 90 degree summer weather and spent a few evenings on the couch popping advil like skittles, but it has been well worth it.
HIIT is where it is at, and you can't get any more hardcore than hills (that said, I retract... I wish I had a prowler).
Good luck and get after it... IMHO - there are very very few guys "overtraining" when it somes to running (keeping in mind it may take a few weeks to get conditioned for it). If my white men can't jump hiney can do it every other day, ANYONE SHOULD BE ABLE TO.