clubmanager

Hi,
Obviously a Newbie. I'm 55 and weigh 215 but am fairly fit; I lift and am on my feet a lot at work. I would like a bike for trips in the 20-30 mile range (75% on bike path/pavement) with a charity ride in September of nearly 40 miles. I have a Trek Gary Fisher mountain bike but my son has outgrown me (at 13 no less!) and needs a new bike so he'll get that one. Point is, I can still use it should I need something for offroad.
I've never been 'into' bikes so am pretty clueless; I'm looking at Gravel bikes as they seem to fit my needs well. Not sure if I would be comfortable on a drop bar bike for long distance but did try one on a Specialized Diverge the other day and liked it for the 2 miles I rode it. Or, do I get another Mtn. Bike and find some less-knobby tires, get an extra set of rims and tires and have both?
Budget is open but prefer not to spend more than $1500. In fact, I'd really like to buy used so nearly anything goes.

thanks a bunch!