2p worth.
I would echo the comments above!
Looking at the results produced by my cousin's husband and son, it seems Zwift is quite optimistic in its power and gradient measurements... e.g. "3W per kilo" in Zwift does not seem to reliably translate to the same on the road.
Notwithstanding, indoor cycling can be very useful in improving strength and fitness. However, if you do do so, I would recommend that you use a properly calibrated trainer and measure your max Heart rate and FTP properly. Once these are established, use these to set up meaningful sessions on the indoor trainer. As you get fitter, your FTP should rise. To translate to the road, it is useful to do decent length sessions with percentages of time spent at tempo and threshold with definite excursions into anaerobic. Such programs should available on most intelligent indoor trainers. (In fact, the only way I can cope with the boredom of the indoor trainer is to use varied entertaining and challenging programs)
With an FTP of 2-3W/kg, no hill in the UK should be beyond you with the proper gearing.
Otherwise, I do recommend regular time on the road with hills providing interval efforts to get the miles into your legs. These rides also need to be contain productive miles with some time spent at threshold - this may hurt a bit but its worth in in the end.
Technique is also useful - pedalling in "circles" is preferred; pedal clips help with maximising power; if you have access to something like a Wattbike, it can give you visual feedback on your pedalling technique.
I am in my mid sixties now. I try to maintain a fitness level of ~2.8-3W/kg with 2-3 road rides (50-100km) and 1-2 indoor rides (~45-60 minutes) a week. I can comfortably climb any hill in the Peak District at about 12-13 m/min (VAM ~700-800). My lowest gear is 30x27. The longest climbs in the Peaks are about 6km rising about 200-300m; gradients ~4% (easy) - ~20% (steepest)
