A tip on working out: Don't overdo it!
As you are just starting out, you'll gain strength and stamina rather quickly. By the end of the second week you'll feel like a superman :-) That is, you'll get in much better shape physically than you used to be and you'll be tempted to push it even harder. Don't!
A mistake I've made countless time is to push it harder and harder until I've got injured and had to stop completely. Think of it as a marathon race, not a sprint. You want it to be a part of your daily routine and not something to be done with after this 100 days.
Slowly increase the intensity of your workouts and take the rest seriously. An injury is something that will mess up your plans completely.
Good advice. staying injury free is key, I noticed my leg being stretched a bit the first couple days and did not push it at the time but put it a bit to rest.
The idea is indeed to brute force a standard healthy lifestyle into my subcounciousness so it fully becomes automatic after the 100 days. I did the eating for 30 days a couple years back which really changed a lot but did not fully fix everything. Next up will probably a year long 365 challenge if these 100 days are not enough to fully do the trick.