The first two to four weeks of exercising on keto are genuinely harder than normal. Your body is used to burning glycogen — stored carbohydrates — as its primary fuel during exercise, and when you remove carbs, it takes time to adapt to using fat instead. During that transition, you will feel sluggish. Your runs will be slower, your weights will feel heavier, and you’ll probably need more recovery time. This is normal and temporary, but it’s worth knowing about in advance so you don’t decide keto is broken when you’re struggling through a run in Sonian Forest feeling like you’re moving through wet concrete.
Brussels is actually a reasonable city for outdoor exercise once you know where to go. The Forêt de Soignes on the southeast edge of the city is probably the best option — proper woodland trails, enough distance to do a real run or a long ride, and mostly flat once you’re in the forest itself. The Parc du Cinquantenaire has a running loop that’s decent for shorter efforts. The canal paths heading north toward Laeken and beyond are flat, which is useful in a city that gets hilly fast once you head south.
Cycling, which half of Brussels seems to do either by choice or because parking is impossible, is perfectly compatible with keto once you’re adapted. For commuting distances, you’ll barely notice a difference. For longer weekend rides, you may want to experiment with your timing — some people find they perform better if they eat a fat-heavy meal a couple of hours before a long effort rather than riding fasted, especially early in keto adaptation.
Strength training tends to work well on keto for most people. Once you’re past the adaptation period, maintaining and building muscle on keto is entirely feasible. You probably won’t be hitting new one-rep maxes in the first month, but after that, most people find performance returns to normal or close to it. Protein intake matters more than people sometimes acknowledge — make sure you’re eating enough of it.
The one area where keto genuinely struggles is high-intensity interval work — sprints, intense circuit training, anything that relies heavily on anaerobic effort. That work runs almost entirely on glycogen, and keto limits glycogen availability. Some people work around this with targeted carbohydrate intake around those sessions, but that’s a more advanced approach and probably not worth worrying about when you’re starting out.
The adaptation period is the main thing. Get through the first month, keep exercising even when it feels hard, and give your body time to sort itself out. It does sort itself out.