Why can't I get the right gearing????

[PpPete]

Or if there is a gym near you with a spin class, try it out for a few weeks. Apart from anything else the, er, "scenery" is usually worth it.
My "relaxed" pace on the road has been in the 90-100 range for a long time but I really struggled to get beyond 120 at the beginning. Now I can hold 140 for a full track and "burst" to 165 for short periods.
No doubt if I'd taken this up before my sixth century I might be hitting 180s by now

 

[RecordAceFromNew]

Certainly there is a lack of compatibility between road and MTB as far the shifters & mechs are concerned (they use different cable pull amounts)
but both MTB and road mechs will usually operate the intermediate sized chainsets that are produced for hybrids and touring.Thus road mechs designed for 52/39/30 may struggle with MTB chainsets like 42/32/22 (and vice-versa) but both are probably OK with 48/38/26 even though the curvature of the mech cage may be less than ideal.

+1.

It is also worth watching out (and prepare) for chainline incompatibility if mixing and matching - Shimano road triple and mtb triple chainsets (and therefore associated front mechs) generally are not designed to operate with the same chainline so one can run out of reach inboard or outboard. Road 45mm, mtb 47.5mm or 50mm.