If want long cardio, best to stick to cycling, if you are talking about soul sucking., esp with the weather here. Stick on a bluetthooth speaker, every day 2hr (even 3hrs possible) zone 2 also can (maybe 1 day off day, also best to have at least 7 hours of quality sleep). (but need halfway break to re-fuel, eg mcdonald's softee is good, now no more Hershey's liao)
You can imagine the amount of base you can build. Even if you dont get competitively fast, you'd still get pretty fast. Definitely significantly faster than those weekend warriors even 1 to 2 decades younger than you on better bikes.