As far as sprints go, I usually only do them when I am training for a fast half marathon race and I try to follow Hal Higdon's advanced half marathon plan. It calls for 400m, 800m, and 1600m speed interval days, but he specifies the pace usually 400 is done at your 5k pace, 800 at 10k pace, etc. I admit I usually do the intervals slightly faster though. It's hard to tell if the structured training and harder speed work did much than just a few interval days and my usual winging it plan... my fastest half still is one that I did without training much, though the spring halfs that I trained for did improve on those particular course times, and only few seconds behind my fastest. And I did achieve my fastest 10k time amidst the half marathon training this spring (still need to put up these race reports). So I do think it helped at least a little. And I hurt a little less after the race... I used to be in pretty bad shape after a 100% effort race for a few days, I still get sore but its noticeably less. I think I'm done with intervals for a while though, going for an 18 mile race on labor day and marathon in October (road ones)... not planning on running these competitively, so just training for the miles. Trying to follow Hal's Intermediate 1 marathon plan jumping into week 8 this week. The 70+ MPW folks... I wish I had that kind of free time. Reminds me when I was first really interested in running an ultra and bought Koerner's book. I think somewhere in the first few pages it said something like training for your first ultra isn't too different than a marathon 40-70 miles per week. I closed the book right there and don't think I've ever opened it up again lol. 40+ is a very rare week for me. I find it difficult to even get to 30 on many weeks, but that's what I shoot for. For example one of the 5 weeks in my shift rotation I work 72 hours, and have about an hour drive each way to work too. My dayshifts I am more stuck in my chair for 12+ hours straight, and so tired and fatigued from lack of sleep and sitting my legs are usually pretty tight after work and have to be careful to just do easy runs. But then I have a week off, and my nightshift week I don't have much issue getting workouts in, so I can get good weeks in but consistency is something I can not achieve working a 5 week rotating shift schedule.