I think you’ll find a lot of it is still there, but the brain has put into deep storage. I learnt hiragana six months ago and thought I knew it 100%. I then gave up on Japanese because I hadn’t found a study from home program that worked and joined the 90% who quit. Now I’m back, got myself a better plan. I relearned hiragana in an hour or two but this time round, I carried on. I use Anki and real kana app daily and not forgetting anything. So I think you should run through the program again but this time you’ll be on speed and a lot will return. It will be hard because “I used to know this once yet I don’t anymore so I suck” sort of mentality.

I also got back into running after a 20 year break. I didn’t try and compete with my formed 20 year old even though I knew I used to post way better times. For eight to ten weeks, it was hard, but worth it. I’m now a better runner than I was twenty years ago, because this time round, I learnt to run properly, train without injury and really enjoy running. If you apply the same to Japanese, you’ll learn fast and better.