You should not rush into to running! Good call on not trying the HR Drift test by running. No non runner should do that. XC skiing will not be a good method either due to the undulating nature of the the terrain and the vast speed difference between the ups and the downs. So, that’s out. A stationary bike can be used but it is highly unlikely to give you the same results as a foot borne test. Rather than waste time on that I’d recommend using the MAF method described in our article: “Aerobic Self Assessment for Mountain Athletes”. That’ll get you close enough to give guidance until you have access to a stair machine of treadmill. As another check on intensity: Keep all your aerobic base training at no more than a conversational pace. Those are simple ways to get started.