MTBs in 1991 looked like road bikes with flat bars.
An MTB now looks more like a motorcrosser, but with out the comfy cushion in place of a saddle.
If the OP has a nicely racey set up on his whippet, he’ll probably be fine, a road bike will be somewhat quicker.
I’d not fancy even 30 road miles on some of the MTBs i see around now.
Ahhh, i now understand the entire recumbent market, people who can’t set bikes up properly….. 😆