Done it 3 times, will miss it this year unfortunately. Great event.
I'd echo Mr Head's comments
- The road climb up Boreas Pass (to Bakers Tank) is railway grade, but relentless. 20-40 minutes depending on your fitness level. So make sure you've got some long sustained intervals under your belt. Don't kill yourself on the first lap, monitor heartrate or power closely. It's easy to get carried away.
- It's not an especially technical course. A hardtail would be fine. Short travel XC bike probably ideal to avoid being beaten up.
- There's a significant hike-a-bike section in the middle of the lap (Little French Gulch). You'll probably benefit from doing some steep hiking/trail running to prepare for that. Your calves will get a workout.
- Tires: It can depend on the year. Sometimes it's blown out and really dusty. This year, it's quite probably going to be wet in places due to the extended snow season. For the Dry years, I've run a Maxxis Ardent Front and an Ardent Race Rear, and I was happy with that (note, I'm not real fast). It's not overall especially rocky, but there are a few rocky sections, so EXO casings or equivalent would be recommended.
- Aid stations are pretty good. You could get by relying on those. I took 2 bottles of mix per lap (2.5 hour laps) and topped up as necessary at the last aid station.
- It can be warm and/or cold due to the elevation. Bring a range of kit. Be prepared for wet weather. It's a bit early in the year for the usual afternoon thunderstorms, but with the amount of precipitation this year, it's quite possible it'll rain. My personal tip is to bring a plastic shower cap...they can do wonders in keeping your head dry and warm when the weather turns at elevation.
Breck is busy at that time, but I haven't found it too unbearable. Just be prepared for some waiting.