Unless something has changed in the last 2-3 weeks, I don't think there are any seasonal race paying multiple millions of EXP for one race. You can surely make more per minute with seasonals, but nothing comes even close to the massive single event EXP reward you get from the enduros. The longer ones give you most of the EXP you need to move up one of the last couple levels in a single shot with full login bonus. Well, except the 24 Hours of the Nürburgring(thanks, PD).
It's not worth it to do enduros for money, but if you enjoy doing them (and they are much more fun than running rabbit-chasing seasonals over and over to me) then it is worth it to do them for experience points... whatever EXP is worth. I don't know if the 9-24 hour ones really pay enough more than the medium-length ones or not, never really studied it since I got to level 40 on one of my multiple Ring-4hr losses long before I ever attempted the longer ones.