One thing to consider is that the more efficent you become at a given movement, the more difficult it becomes to induce an adaptive response to training. If all you do is run for 2 hours at 150 BPM after a couple of months your fitness will stagnate and likely decline a bit. I dont want to imply any LSD prejudice so I'll point out that if all you do is tabata intervals the same thing will happen. Throwing in a bit of gym time in the off season will help you to break that plateau. This is one of the main reasons crossfit is such an effective progam it never allows you to become efficient at anything. It forces an adaptive response to each work out.
Another thing to consider is that maybe weights are'nt that helpfull when it comes to steady state aerobic events such as marathon and XC mtb. Road racing is an entirely different kettle of fish because it combines random bursts of extreme power with steady state aerobics. Some of the most successful XC mountain bikers I've known and trained with use almost no weights at all.