Hmmm... Well 20 miles a day would be a bit steep for me, but I am older then you are. The ground you'll have to cover is pretty rugged, but not as sever as say the Presidentals in the Whites. Some steeper grades. Gets cool even in the summer time but nothing life threathening. I havent done it end to end, nor have I hiked the Green's in years, best to check with the Green Mountain Club.