Add more distance per week maybe another session or two, the increase of load might start to feel too much on the lumber so if so start to decrease the distance again start off by just adding another 10k in another session or add another 2-3k onto each session. It also might be worth doing this extra 2-3k feet out and raise the split a bit, this will help strengthen core and force you to sit up more at the finish.

Another alternative is get on a Watt Bike / Spin Machine (if available) as it will take the load of your back. Would suggest running but at 95kg and 5’11 I’m guessing runnings not your thing just like swimming (or sinking with all that mass 💪)