Walking ten whole miles in this heat at a plodding pace is unbearable.
Literal
This heat [in-に] 10 miles [even-も] plodding walk [nominalizer-の] [topic-は] unbearable.
てくてく is a mimetic for walking at a steady, plodding pace — not fast, not slow, just trudging along. も after the distance emphasizes 'as much as ten miles.' たまらない 'unbearable / can't stand it' is a common complaint expression. The sentence packs a vivid physical sensation into a compact structure.