My right knee is completely fubared, I can only run up to about 3 miles in one session. 10 k's to half marathons I run/walk as best I can and always adapt the plan to how it feels. Marathons I walk the entire way (although I've never done a stand alone marathon). I've also had to pull out of 2 IM distace events after my knee has gone on me.
As long as you are prepared for a long day, willing to adapt to how things feel and put up with well meaning people encouraging you to run when you know you either can't or don't need to then it is perfectly doable! My marathon PB is around 6:15, and IM PB (from a different race) is 15:59:11!
Well I was at Sherwood Pines a week ago and the trils were dry and fast. We've had very little rain since then and the forecast is for it to be dry, but cold this weekend! So I've either just jinxed it of it's going to be a nice fast race on the MTB!!