Blood vessels and capilaries are slower to develop within muscle tissue because the muscle tissue is very dense and tough.
The rate at which you can expect gains varies widely on factors such as your genetics, the amount of time you rest in between hard rides, nutrition, etc.
In my professional opinion (as both an N.P. and an old-school MTB'er) I would say your muscles lag behind your cardio by a good 4-6 weeks at least.
Cardio fitness is actually MUCH more complex than the original poster says. It consists of many things, such as the strength of the myocardium itself, the level or amount of erythrocytes in the blood, the number and capacity of blood vessels and capillaries, etc etc
All these things take TIME, which is why resting is so important.