100+ miles is an optimal ideal if you have the mechanics and joints to pull it off. I was once a 110 MPW guy in my 20's and starting paying the price once I reached 35. My theory is that the heart and lungs can't tell if you are running, riding, or using a gym machine. If you can replace junk with an equivalent amount of cross training, you can reap the aerobic benefits without the punishment. (Realistically, most runners can't do 100 as their bodies will not allow it.)
I still train everyday, you need to. For my log, I approximate every 10' of riding equals 1 mile of easy running. I have no scientific rationale for that only that my HR is about 10% lower on the bike (thus more duration needed) and my weight has stayed the same. (60 MPW vs. 35 MPW + 24 "bike miles"= 59 "miles" of training) The big difference is that I feel much more snap in my legs. I try to maintain a 85-95 RPM cadence while riding to maintain leg turnover and not get monster quads - this I feel is key.
For the marathon, I plan to max out at about 60 MPW with about 30 additional "bike miles" each week. I'm intrigued by the Hanson's plan laid out in last month's RT: lots of specific quality with ample rest. Replace rest with some XT and some recreational runners could really fly due to inproved fitness and strength. I'm never going to run 2:10 so I'll experiment and see if I can match the 2:25 - 2:30 clockings of my youth.