When I ran/jogged, I found that some days I just put out less. Never figured out the why, it seemed sort of random. At that time I knew a guy who ran trails weekly (long ones, like 50-60 km total). He told me it's normal to have day with more stamina and days with less. He was extremely fit, had been doing that for years, and had days when he couldn't do the longer ones, or chose a shorter second trail because he got tired sooner.

Point is: your fitness level is going up steadily -- the days when you perform less are not an indication of your overall fitness, and still contribute to making you more fit.

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It's great that your weight went back down. Love when that happens.

I wish your friend well, too.