Gaining steam

Since I've been back in Northamptonshire, I have walked every day (including the day I returned) but since Monday, I have walked to the neighbouring town and back, which is about a five-mile walk in total. I have completed the walk every time but it exhausts me more than it has done previously; however, I am slowly regaining my steam to endure medium-length and long walks again. I'm bringing 3-4 bottles of water with me on each occasion, as I need the hydration, not just to restore my fitness but also to counter the heat and humidity.