Over the course of 21 hours, Julie Nisbet, a runner and mother of two based in the UK, ran her way through 69 grueling miles of an ultra marathon — despite the warm weather, she was focused on making the distance and applied sunscreen twice throughout the entire day.

Nisbet, from Shildon, County Durham, England, began the marathon at 7 a.m., prepared by months of training — but sans sunscreen. At her first pit stop three hours later, she slathered on SPF 30. “It was only 10 a.m., and the sun wasn’t too high in the sky, so at that point I still felt okay,” she told Buzzfeed.

At 27 miles, she felt the skin on her calves (she was wearing cropped leggings) begin to tingle and burn and reapplied for the last time. For the latter half of the marathon, she didn’t reapply at all. When Nisbet crossed the finish line with 69 miles in her wake, small blisters dotted her burned legs — exhausted from her long-distance journey, she took a nap, hoping the pain would go away, and visited a local urgent care clinic when it didn’t. Doctors bandaged her up and told her to return the next day.

The next day brought agony — the blisters had swelled into throbbing, fluid-fill sacs that bubbled out away from her skin. The rest of the flesh on her legs was a bright, raw red. Nisbet took to Twitter, where she chronicled the painful injury in a series of photos and tweets, describing how devastated she felt that her time of deserved celebration (an ultra marathon is an ultra feat!) was overshadowed by such crippling pain.

Nisbet returned to the clinic frequently that week to have the blisters drained and bandaged to prevent infection — the thought of enduring that process over and over, is enough to make anyone squeamish. “The healing process has been painful — the burning and throbbing as the blisters have filled was a horrendous experience,” she told Buzzfeed.”The pressure in my legs when standing and walking has been also been agonizing at times.”

Today, just over a week later, Nisbet is on the mend. Now that the blisters have flattened and the agony has subsided, the runner is the first to admit her mistake. “I’m under no illusion of how negligent I was. Our body is a wonderful and resilient thing, but it also needs constant care, and I didn’t pay enough attention during that race to reapply suncream,” she explained. “It’s been a harsh lesson to learn.”

Nisbet may have learned the hard way, but you don’t have to. When out in the sun, be sure to take the proper measures to protect yourself and your family:

1. Apply sunscreen at least 15 minutes before going outside.

2. Check the bottle’s expiration date to make sure the “active ingredients” are still active.

3. If you need to buy a new bottle, shop from this list for the strongest protectors.