Mr Viper
I think your thinking is backwards. If you feel your quality component is too high then reduce it - don't increase your easy mileage to justify it and make the sums work. If I were you I'd increase volume / quality gradually and in proportion (which means at your current mileage reducing it) whilst listening to your body and see where you end up. I doubt it will be 70, but if it is then at least you've done it methodically.
2 runs.
In terms of training effect (for a marathon) one a days are better than splitting it up - assuming you are recovering sufficiently.
The reason I like 2 a days are as follows - and they have nothing to do with the goal of increasing mileage although that can be a byproduct. But the 2 a days would be additional for a marathoner.
1. On quality days. There is an Italian study that showed that percieved effort is lower in a session done 4 - 6 hours later than a run of about 30 minutes compared to no run at all prior. Apparently the earlier run results in more production of HGH which can reduced perceived effort or perhaps actually true effort. (A sleep of 30+ minutes also results in more HGH production by the pineal gland as well). So the purpose of this would be to improve your quality workouts. Anecdotally I have found a run of 2 - 3 miles sufficient and you will note many Kenyans run 3 times a day - each run seperated by 4 - 6 hours.
2. The day after quality days. Renato Canova has described his research that a run the following day after a quality workout helps to reduce resting lactate levels in the blood to a level lower than if you hadn't run. He says these should be done @ 80% of MP.
On these days I try and maximise the time between the early run and main easy run because maximising rest and recovery is the priority. It's important to remember that making the 2nd workout easier and maximising rest are two different things!
* All the above assumes you are fit enough of course!
Edited: 30/04/2010 at 13:26