Went out planning to do 3-4 miles and just take it easy, took it very easy but my legs felt heavy and it felt like my heart just wasn't in it (literally, like it didn't want to beat any faster). After doing a hill and not enjoying it I abruptly decided to stop and walked the rest of the way home. Sure enough my heart monitor showed my heart wasn't getting up to its usual BPM, no idea what was going on, but I do struggle in warmer weather and it's 25C and humid, probably put it down to that.