I always used to use road biking for MTB training.
Do some 5-25 mile time trials (hilly’s are good) to build up your power.
If you are after losing weight then you need to be doing 2 hour plus rides of more than 30 miles at a lowish intensity. Heart rate below 130 at all times. Ideally 4 hour rides of 50-60 miles will see the weight drop off!
Then I always found road biking to be a great tool for warming down after a heavy week on the bike. If you feel a bit tired but not dead to the world then go and ‘spin out’ a leisurely 10-20 miles to loosen your legs up a little.
You could look into this in great scientific detail but biking isnt really rocket science. Know the basics of what works for you and just go and ride. When you feel tired then rest.
Matt 🙂