At the risk of opening up a whole new front of disagreement..I think that road is more likely to improve fitness due to the easier access to the type of surface inductive to maximum effort, otherwise known as a road.
Out the door 2, 3, 4 hours not stop uninterrupted effort at whatever level you want is accessible to most on a road bike. Thats more difficult to achieve on the MTB (is for me anyway) which whilst I can string some long loops together more of it will be backing of for the loose surface and all the hazards that make it fun.
I also don’t think you can replicate the sensation on mtb with slicks, road bikes only really come into there own at 19/20mph+ even better if thats in a group. The fitter you are the better it gets, which is usually encouragement enough.
Not being a little fit would be like sending me down Fort William Downhill and concluding it is crap cos I had to walk most of it.
FWIW thats my experience of being a regular MTBer and Roadie.