According to my podiatrist

My tendonitis is actually secondary to a heel spur and plantar fascitis. I'm to do stretches throughout the day, ice them especially at night (including rolling an icy 20 oz. water bottle under my feet to massage them (it may be as effective on the pain as ibuprofen, which helps but can't be used long-term). I am not to go barefoot even inside my apartment, but wear shoes, even if they're my Birkenstock knock-offs, something with support. My doctor wears crocs, for example. Just socks or houseshoes are also verboten. We're going to check if my insurance will pay for orthotic inserts, as my feet suffer from pronation, where the foot tends to rock to the outside. It's one of the reasons I tend to have ankle sprains, apparently. Then there's the heel spur issue. In other words, my feet's structure is such that I'm a prime candidate for these problems. Age affects the tendons as they become more brittle, so that is part of it. My weight and standing so long at work have aggravated the problem. But happily, there is no boot or crutches in my immediate future. If this does not help, cortisone shots into the tendon may relieve the pain. I'd rather not do that, though, so I'm going to try everything else first.