Well, Rubmyster covered nearly everything I was going to say.
I would second that 3 hours is waaaay too much. That reflects his mental posture more than anything, it just says he's really sore and wants work done equal to that soreness. Sounds like instant gratification/weekend warrior type.
A one or one and a half hour session every other day is what I would reccommend.
If he didn't train for a MARATHON (are you kidding me?) it's his problem, don't take it on yourself, there's only so much you can do. If you try to "fix" him he will vampire all your energy very fast, especially in a 3 hour marathon massage seesion. I'm very clever.
The best you can do is circulatory work to facilitate recovery. ROM, vibrations, petrissage and effleurage. Don't do any stripping, that will set him back (learned that the hard way).
If you do agree to a 3 hour session (bad idea IMHO), then agree on a break in the middle.
Remember, there's only so much his organs can process, more work will just be more to process.
He also needs to do some intense hydrotherapy. Ice bath ASAP, not full body, just legs, then just sitz. Then a few days later, some intense ice bath on the legs, wait 2 min, then into a really hot (not crazy hot) epsom salt bath for 20 min, then back with the legs into the ice, then 2 min, then back into the hot bath.
Think circulation and lots of drinking water for this.
This situation has all kinds of red flags in my view. Sounds like he didn't take care of himself and now he wants you to fix him. Show him how to take care of himself with water and plenty of rest (sleep and REST), and how you can help him along with the circulation work. Don't try to fix him, only time will do that for him.
Hope this has been useful, good luck and let us know.