Buy a cheap e-bay cargo trailer and at least once a week ride with a few kg of ballast in the trailer. It’ll strengthen your climbs by adding resistance and if you make the effort to ride stood up regularly to blast up climbs, you’ll add some upper body & core strength to the equation. Then do some more core and upper body work as soon as you get home; power delivery is always better when you are pushing against a stable set of core muscles.

Heavy Tyres, Supertacky, Low Pressures, Full Suspension. Duct Tape your gear shifters so you can’t use them.

go a different (longer) route.

Buy a cheap e-bay cargo trailer and at least once a week ride with a few kg of ballast in the trailer. It’ll strengthen your climbs by adding resistance

adding weight or resistance isn’t going to help because climbing or riding or anything on a bike requires “an” amount of watts. If you add weights or resistance the amount of watts you can produce isn’t going to change you’ll just a) get tired more quickly or b) go more slowly.

The only way to get faster/stronger/longer is to increase the number of watts you can do, and that takes aerobic fitness, which is best done by interval/sprint training. As the rules point out “It never gets easier, you just go faster”

adding weight or resistance isn’t going to help because climbing or riding or anything on a bike requires “an” amount of watts. If you add weights or resistance the amount of watts you can produce isn’t going to change you’ll just a) get tired more quickly or b) go more slowly.

Riding fast at a certain power is different to riding slow at the same power though (e.g. high inertia riding on the flat compared to low inertia riding going up hill.) The former can feel more demanding. If you want to get better at climbing then you really need to go climb some hills.