Running is too easy

I got in two hours and fifteen minutes on the bike, with as many hill as I could find.  Then I hopped off and ran 3 miles.  The running feels too easy when I get off the bike.  I know there's a HUGE difference between 3-5 miles and the marathon after a 2.4 mile swim and 112 mile bike, but every time I do a brick, it just feels easy.  I'm feeling really good about the IMTX run.  I still have work to do on the bike (and in the pool), but I'll finish this race strong.