Engaging in only one type of sport all year is difficult and often impossible.
Reasons are mainly weather, travel time to the starting point, monotony of the same hill and resulting boredom.
It's good to choose several different sports where different muscle groups work and choose a suitable one according to season and weather. It can often be nicely combined: triathlon disciplines (running, road bike, swimming), hiking, MTB, fast walking, nordic walking, last but not least fitness for rainy generous weeks.
The initial 3-5-hour weekly trotting will be a good base for later possible extension to more hours. Most important are continuity (no more than 2 day breaks), moderation of training, proper nutrition (important meals are breakfast and post-workout meal), rest at least 2 days/week (walk is rest) and maximum enjoyment in training.
Conclusion: let it not be a one-year fly, let it get under your skin and become part of everyday life, as analogy it's not 100m run, but marathon (long distance run). Good luck.