Hope you are wearing the 50+ jersey when out on your daily exercise walks into town for Pie.
Sounds as though you want the wider tyres for comfort and shock absorbtion- instead of less rolling resistance and take it you will not be using them longer than necessary.
Not used either tyre- but the Tandem is 26" wheels and I used one of the Marathon slick tyres for quite a while. Finally had to change them and went to a conti and did not notice any difference in Ride quality. What I did notice was puncture resistance. Those swalbes are a durable tyre that must have a puncture belt built into them.
Sorry about the Gutters needing cleaning- but keep the rest of the body in trim before you get to them. Torso and upper body strength should be kept in trim and don't forget to exercisde the legs- in particular the quads. All that being supported on a walker is not going to help on leg strength in 6 weeks time.
And on the walkers- My local Seaside town is famed for the Walking frame races as the residents of the hotels rush down to the pier after breakfast to get the best chairs for the days resting. At your young age and it only being a temporary usage- you should be able to get the best seat every day.