^Exactly this! I was a figure skater for most of my life and played club Ultimate Frisbee in college so most of what I was doing was sprinting and cutting, high-intensity for just a few minutes at a time followed by a break and more of the same. Doing a 5K was extremely out of reach for me for a really long time until I started slowly building up the mileage and took my tempo down a notch. Building slow-twitch muscle strength takes some time. If you're running 5 days/week you might also consider running 4 days/week instead to give yourself more recovery time before a longer run, especially if you run harder but shorter during your other days. Another thing I notice when I do my runs is that if I don't hydrate enough the day before I feel totally wiped by the time I hit mile 2 or 3. It's also easier for me to run longer if I am not listening to fast music because it's natural to pick up the pace when you're listening to a fast song! When I switched from listening to my favorite high-tempo Bomba Estereo songs to chill Ben Howard music it was a bit of a slog at first but now I look forward to the vibe and the place in my mind I can disappear to during my long training runs. FYI I'm early in my running as well and haven't yet done a full marathon; I was supposed to run Big Sur Marathon this April and pushed through all the training (including two 20-milers) but did not want to push myself to do a full on my own at this time. I got into running like 6-ish years ago and used to hate running because I only ran on roads; as soon as I signed myself up for a trail 5K and performed well that's when I decided I actually liked it. If you're running on roads but have trails accessible near you, you might switch to those for a bit so your mind is taken off the miles and focused more on navigation/your footfall/what's around you. Running is like 80% mental and 20% physical--maybe that's an over-exaggeration but I think others would say the same.