i recently started running again and am on a more structured plan of 50miles per week and 2 quality speed work sessions.
I couldnt see how i could fit 50miles into one week, it looked impossible but the advice i was given was do one long run approx 15miles and do the rest in whatever length you can do.
So i run a 10m on sat morning, 15m on sunday and monday to thursday i run 5miles in the morning each day. So this running doesnt get in the way of work/wife/kids time i get up at 6am.
Now after on 4 weeks i cant imagining not doing this! It doesnt feel like a chore.
Its time on your feet that counts and dont feel every session has to be run at your fastest speed.
rgds your goal goal of dropping 45mins off your PB, thats nearly 2mins per mile with is a lot. A good guide is can you run 5miles now at this faster pace? My advice is dont get caught up in big leaps in reducing your PB, find a good pace, get the miles up and it will happen naturally