Personally I think 65 mpw is fine. Depending on how long you've been doing such mileage may be why you feel lethargic. If you've only just started to up your miles then you will be tired. I'm 9 weeks into building my mileage up and I'm only just starting to notice the real benefits. I do the same 4 quality sessions over the month and next week is the real show as to whether my training is going well. Over the month with upping my mileage though I've seen my 5x1000 session on the grass improve by over 10 seconds a rep. Provided you can maintain your mileage you should notice a difference really. 65mpw isn't mega mileage and recovery should be possible. Listen to what your body says if you do feel shattered one night then theres no point in going and murdering yourself for it. Adjustments to training do take along time to get used to properly. You have to remember the 3P's is 24 miles in length but time wise should take you longer than a Marathon unless your marathon is really slow. Whatever mileage you put in make sure its consistant. My training last year was 10 one week then 60 the next then 30 you get the picture. This year its 45-50 and only had one blip in there at 26 or 28