Why not cover the whole region? Start either in Mexico or in Panama City and work your way to the other end. Backpacking the Gringo Trail through Central America is doable in 1,5 months minimum as the region is quite compact, though you can easily stretch it to 3-4 months if taking it slow (or when seeing more of Mexico than just the southeast).
The Ultimate Guide to Food and Adventure in Guatemala. This free guide highlights culinary experiences and various ways to actively explore Guatemala. Five different areas are highlighted: Antigua, Lanquin, Livingston, El Remate & Lake Atitlan