Training for Marathon in Fall/First 2 Tris in summer

OK...I am doing my first 2 Tris this summer (Sprint on 7/12 and OLY on 8/30) .  I am also doing a Marathon on 10/18.  I am doing the 2x balanced OLY BT program and I am currently in week 1.  I just did a 25k at and 8:11 pace and gave an all out effort.

How should I modify my TRI training to prepare for the Marathon?   Should I do 2 runs during the week as a part of the TRI plan and add a long run starting now on the weekends?  Or maybe 16 weeks out from the Marathon add the extra long run?   I feel that aerobically I was just fine for the 25k, but my thighs were on fire at about mile 14 which took my 8:07 pace until that point up to an 8:11.  Are the long runs (>13-15 miles) necessary all summer long, or if I start at 10 about 10-12 weeks out and work up to 20-25, will that be enough.  I was thinking of a 3:45 Marathon as a goal.

Any thoughts?