How to regain attention-span before starting PhD?

I’ll be starting a PhD in the social sciences this month. Beginning in late December and lasting until this June, I had a series of serious health issues that landed me in the hospital several times and made me essentially home-bound every day. Additionally, I was only able to eat very little, non-nutrient food (think soup and rice). This made it almost impossible to sit down and read/write, much less think through the questions I hope to tackle during my PhD. Thankfully, I am now doing better and am on medication that is helping. I’ve also been able to eat more calories and nutrients. However, as I’ve started to read again, I’ve found it really difficult to concentrate. I’ll read for a little, get distracted, and ultimately cannot remember what I’ve read. Does anyone have any tips on how I can build my attention/stamina back up before the program begins? I haven’t had this problem in the past, so I’m not sure how to overcome it. Thanks!