Originally Posted by cubewheels

It really depends on your route if it's safe to ride fast. If it's mostly low density traffic roads (and the traffic is quite predictable) and you think it's safe to ride fast in there then by all means, get a fast bike if you think it's more fun.

But if your route is mostly in the city with medium/high density traffic, sharing the streets with pedestrians, motorists, couriers (that make lots of random stops), etc, it would be dangerous to reach high speeds in a bike in such conditions (bicycles have very poor/dangerous emergency braking performance/handling compared to cars or motorcycles, thus, unsafe to use at high speed in chaotic city traffic, ironic exceptions are cruiser bikes with low saddle height). A hybrid or MTB or even a fatbike is better suited to such conditions (so that even if you ride slow, you'll still get a very good workout because an MTB or fatbike has considerably greater drag than a road bike.

I'm actually training myself at the athletic level with cycling. But since most my route is along busy city streets, I'm using a MTB that is setup with road bike geometry. The tires are stock, wide knobby ones for higher drag. To further increase drag, I installed full fenders, large mudflaps, and train with very loose hoodie jacket. The bike is 40 lbs in the training setup.

I did that so I can train really hard and avoid getting too fast (in the city streets) at the same time. It's actually quite effective if you are unable to train along low traffic roads.