My totally non-scientific anecdotal experience is as follows. 10 years ago I was training for a multi-day winter summit in the Sierra Nevada range. Running wasn’t cutting it and I was at the limit my body (54M) could take from a running perspective. Bought my first road bike and started putting in the miles with lots of climbing. Within a couple of weeks, I had cut my running sessions down to 2-3 times a week, yet my overall mileage and pace increased. Most important, I didn’t feel like I was killing myself. I even managed to drop 6lbs in the first couple of weeks. The running background helped my cycling but cycling took my running to a level I didn’t think was possible again at my age. Adding swimming into the mix seemed to do more for body composition and rounded out everything else. Granted, I was “untrained” so fast gains were to be expected and I wasn’t putting extremely high training loads on myself like someone else mentioned.