Taper Question: Running Boston then doing Kansas 70.3

Here we go ST,

Did a 18 miler at 7:30/mi last week and have 18 days until the Boston Marathon that I qualified for in June with a 03:02:35 split at Grandma’s Marathon. Anyway, since then I have signed up for Kansas 70.3 as my first long course tri after doing doing some sprints and such this past summer. My taper question is: Can i sneak another long run in this saturday 4/5/14 (20 Mi) and then recover and cut down milage before the marathon on 4/21 or should I start my taper this saturday with a 12-14 miler instead? My run is my strong point followed by the bike, and swim last of course but it is coming along nicely. I will recover on the bike and in the pool following the marathon for a week and get my legs back underneath me with a few light 5k jogs the following weekend but when should I start my taper for the 70.3? 2 weeks out? The Kansas 70.3 is on 6/8/14 which is 48 days between. Share your thoughts. Hoping to go sub 3 at boston and have some personal goals for the 70.3 such as 1:25 half marathon. Enlighten me!

Cliff notes:

2 or 3 week marathon taper?
70.3 taper length?