Me (if I find out where to go when I fall off the train at Euston) Not used to the big city
DW and I joined at Green Park - which is accessible - and then left from Westminster (again, accessible). About a mile and a half walk. The procession goes right past the entrance to GP tube, and it's easy to slip in.
(The more laid back protesters will have spotted that it's possible just to get to the finishing point at Westminster without actually marching
)
DW and I did two marches last year ... it's 50/50 healthwise atm.