Buying a road bike was a revelation. I go through phases of riding more on the road and more off road.
Can't beat a sunny club run at the weekend for banter, scenery and excitement. Does wonders for your fitness as I find road rides are more continuous effort than the stop start nature of mtb group rides.
One thing you need to ask yourself is do you like hills? Its a psycological jump when you stop trying to avoid them and see them as a means to good descents and start to add them in to challenge yourself. If you ride on the road it's all about the climbs IMO. Sprints are fun, breakaways frowned upon on a friendly run morning run chain gangs can be exciting too. cruising at 35mph on country lanes certainly focuses you as much as any singletrack descent.
Embrace the lycra, it's more practical and comfortable. I tried to avoid it until one wet long ride which was just unpleasant in baggies.
Join a club, group riding is ace, they will know the best roads, routes and cake stops.Mine is friendly and the top level guys ride at the back and push the slower ones if needed to keep them in the group on hills. Pace is kept steady with groups splitting off to do longer more challenging bits.