When I rode with non-cyclists, or cyclists coming back from time off or injury, I'd ride my mountain bike. One racer, I think he's substantially stronger than me when he's fit, asked me if I wanted to ride with him. He just started riding after 10 years off, and asked me to ride my mountain bike. It was perfect. I could go as hard as I wanted and it was all good. At some point I told him I wanted to open it up and did an effort on a windswept beach road. He was on my wheel fine, and I got to do a really hard effort. That ride led me to change crank lengths as my mtb had 175s, my road bike 170s, and I went really fast into a cross headwind on the mtb compared to the road bike in a mostly tailwind.
I've done the same when riding with families on tandems, with the Missus and some recreational cycling friends, when riding when the Missus is running, etc.
My mtb is set up as a road bike, with a long, low position, road pedals (I have Keos on it). I switched out the 2" knobby tires but ironically haven't ridden on the road since, except to ride to our storage unit half a mile away. The 2" knobby tires work well on the road at max pressure, 60 or 80 psi.
The mtb has lower/easier gears, forcing a lot of pedaling when the speeds are higher (like on downhills, or faster flats). But otherwise it rides like a road bike
Long time ago at school an older kid (grad student looking) rolled up one day with big boots, jeans, jacket, etc, a book bag full of text books (it was gigantic), on a mtb, asked if he could join us on our ride. We hesitated but agreed, as we were going on a "harder" group ride (we were 3s and 4s). To our absolute shock he stayed with us for something like 45 minutes of short steep hills, attacks and chases, etc etc. Finally he cheerfully called out that he had to peel off and that it was fun. We were all a bit shell shocked that he stayed with us so easily. He came to our next team meeting and introduced himself. That's when I realized he was a Cat 1 / Pro from the area, he'd stopped riding because of back issues and was now starting school a bit late, Apparently that power / strength really doesn't go away. To give an example, he lapped the Junior field in the state RR on a 5km course. It looked like he was on a motorcycle, he flew by us so fast. He got 6th or something at Nationals that year. As a Senior raced briefly for Oldsmobile I think, and I think also for Mengoni (although every strong rider in the area seems to have ridden for Mengoni).
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"...during the Lance years, being fit became the No. 1 thing. Totally the only thing. It’s a big part of what we do, but fitness is not the only thing. There’s skills, there’s tactics … there’s all kinds of stuff..." Tim Johnson