I do road for distance, turbo intervals (and bad weather training), gym strength and swim for relaxing and easing off good cardio and rest.
Turbo intervals and hill work is good for power
Long road cardio and distance
Pool relaxing and easing off
Gym back arm strength
Dont discount the value of rest and recovery
add some skills days on the MTB
Diet, eat sensibly
being fitter overall will improve your speed and endurance
Common myth is that you must kill your self each session, wrong, training is progressive intensity with repetition.
Build a good base you can maintain, measure it KM, time etc then improve on that with goals.