As you get fitter add more full two litres water bottles to the pack.(until you reach 10kg and 30km round trip) That will fix the legs and cardio side of it.
Nature has their own '10kg of water bottles' they are call hills, well specifically uphills.
Find some good ones and you will feels the '10kgs' without it actually being there…
Keep up the walking Tony and make a plan to improve from that base - most importanly, learn how to listen to your body. When it feels 'good' push a bit harder, when it doesn't don't be afraid to take it easy. Better to come back the next day when you feel good than to push through and do yourself and injury which will really set you back. Knowing when to rest is almost as important as knowing when you can push.
Good luck [not that you will need it].
VMX42