cycling would certainly work, if you are vigilant about HR (it is very easy to let ego get in the way on a bike. usually MAF HR limit is slow on a real bike until you get efficient)
Obligatory question, why you want to gain some form of endurance, what is your goal? Specific event, health, GPP endurance?
MAF HR is actually a control for endurance people who are used to go too hard for too long, so it is a "hold back" rate rather than a "you need to get to this level to have benefit" rate. If you have no specific need for higher speed endurance, rucking together with some S&S style swings or snatches is all you need! No need to chase a number, especially one that is a speed limit
on rucking... increase the load slowly, but don't forget to increase duration! Lower HR but longer duration, almost the same adaptation (maybe better, duration in LSD work usually has much greater carryover to anything real-life than higher HR)
that being said, my preferred option to supercharge rucking is to ruck with a KB, every mile or whatever stop, unpack your KB, and do a few sets of 5 snatches or swings or C&J, anything power-based. Stick to low reps with plenty of rest (A+A, feel ready for the next set, you're not chasing HR or a burn here), do a few sets (enough to get you to feel good), pack your kb back in, ruck, rinse, repeat!
This works well for running as well, with the obvious limitation that you would do loops around a KB instead of lugging it around
I find this keeps your HR up (but not excessively so, usually quite well around MAF limit) quite nicely, gives you mental fortitude, breaks up boredom, and gives you a very good conditioning effect! sort of like the S&S of LSD
Kudos of course to
@aciampa who got me onto this type of LSD work!