Hi and welcome to the board!
3.5 years old is prime of life for a Labrador :-) He can get as much exercise as you and Gus want, as long as you are not a 'weekend warrior', and cram a lot of activity in on the weekends and do nothing during the week. If you are planning on upping his exercise a lot, or start running with him, or strenuous hikes, start out slow and build endurance, like you would if you were in training. Otherwise, as long as he is healthy and fit, and not grossly overweight, go for it.
I have a ramp for my Sienna, and Tundra truck, or I use a "paws up" command and boost them into and out of the back, into their crates, so they don't stress their joints jumping in and out at will as puppies. I make it a controlled entrance and exit from the crates as adults, and they are so used to either the ramp, or my easing them down and out that they let me, and that's fine with me.
When I first got my ramp, I followed the directions, of letting them check it out on the ground, then encouraging them to step/walk on it, then we moved to the car, with two people, one on each side and leashed the dog, to keep them on the ramp, with lots of praise and cookies for doing it right. Puppies just emulate the older dogs now, and just follow them right up the ramp as youngsters.