Very anecdotal, I did several of the Grand Slam 100s and Ironmans and was in great shape. Then I built a company, which was my sole focus for several years and I only ran very sporadically and gained probably 20lbs from my race weight. Then as work started to level out and became successful, I was able to get back into running. Signed up for a 50k one day with no real training and successfully completed it in reasonably good style, though I wasn’t competing for podium like in the past.

It took several months of training to work back up to long runs and increase my mileage, but was able to qualify for Boston off of that.

So anecdotally I lost a step after years off but came back strong without much training.