pepe, the simple answer is that the more you do, the stronger and fitter you'll be which means the longer you'll be able to keep up with a faster pace.
The upper limit on this level then depends on your fitness, lifestyle, time, and nutrition. Ordinarily most runners won't do more than 5-7 hours of exercise a week before getting tired enough tha it affects next week's efforts.
I'm happy with 3 days a week for those who can't fit in more, but as others have said, those 3 have to be good sessions. LSR, spped/hills and tempo runs are the key runs, so you'll probably not do too badly. 3 x 20 milers is a good pre-26.2 base too.