My biggest suggestion is to relax and not push it. If you're not feeling up to a park on a given day, stay around the resort and rest or explore the resort. If you're staying longer than a weekend, it may even be a good idea to plan in some days to be spent at the resort to make sure you get some down time. On days when you do go to parks, go by your schedule and your speed. You may be able to do more if you get up for rope drop, take a break in the day, and go back in the evening, however, if you're not used to getting up and moving that early in the morning, there's no point in adding to your stress. You'll also just feel more rested if you're going by the schedule you're used to. If you need to take breaks you can either go back to your resort, or use First Aid to lie down.
I would highly suggest renting an
ECV from an offsite vendor. Using an ECV will let you conserve your energy. Most rides have queues that are mainstreamed so you'd just bring the ECV through the queue with you. A few you'd need to switch to a wheelchair. You can also always park the ECV and walk through the queue, but if you're trying to conserve energy using the ECV more will help with that.
Otherwise, make sure you're drinking enough fluids and eating enough. I don't know what chemo you've been on, but making sure to have a hat, sunscreen, and loose clothing that covers as much as possible will help if it's a type that can increase photosensitivity. If you're not feeling up for rides, WDW is a great place even just to sit and people watch. There is always something going on to watch.