My 7 km commute leaves me wrecked and takes far longer than the map said — is it me or the bike?

As someone who did exactly this: the fitness component is real, and it arrives faster than you expect if the bike is not fighting you.

The order I would work through:

1. Tyre pressure. Today, free.
2. Saddle height. Too low is extremely common on second-hand bikes and it wastes most of your leg. With the heel on the pedal at the bottom, the leg should be almost straight.
3. Chain lubricated, brakes not rubbing. Spin each wheel and watch whether it slows quickly.
4. Then, if the hills are still forcing you to walk, change the rear sprocket for a bigger one. On a single-speed this is a cheap part and a ten minute job, and it transforms a bike that is geared wrong.

If all of that is done and it is still hard, then it is fitness, and three weeks of riding four days a week will change it noticeably.

The time discrepancy is worth separating out, because it is probably not about effort at all.

Mapping apps assume a steady average speed and mostly ignore what a city does to you: traffic lights, junctions, waiting to turn, pedestrians, pausing at every road crossing. On an urban route the stopping is often a third of the elapsed time, and a new rider stops more and accelerates back to speed more slowly.

So 45 minutes for a 25 minute estimate is completely normal on a city commute and does not indicate anything wrong. It will drop as you learn the route — which lights you can catch, which side road avoids the bad junction — usually by ten minutes or so within a few weeks, without you getting any fitter.

Also, getting off and pushing is far more tiring than riding. Two short walked hills will account for a surprising amount of how wrecked you feel.

Walking up short hills on a flat-ish 7 km route is the detail that answers this: it is very likely the gearing.

Single-speed bikes are sold with one ratio, and second-hand ones often carry whatever ratio the previous owner liked. A lot of them are geared high — fine for a fit rider on flat ground who wants speed, brutal for anyone starting out or meeting a gradient. There is no way to make a too-high gear easier except with your legs.

Two things to check before you conclude anything about your fitness:

• Count the teeth on the front chainring and the rear sprocket. A ratio around 2.5 or below is friendly; well above 2.8 is a racing ratio and will hurt.
• Check the tyre pressure. This is the single most common cause of a bike feeling heavy, and it costs nothing. Underinflated tyres can easily double the effort. Pump them to the pressure printed on the sidewall and ride the same route again before changing anything else.

That second one has fixed this exact complaint for more people than anything else I know.

One route note: the app's suggested path is optimised for distance, not for how pleasant it is to ride. It will happily send you along a fast road with four lanes of traffic and a hill, when a slightly longer flat route through side streets exists.

Worth spending one weekend afternoon riding two or three alternatives with no time pressure. The route that is 800 m longer and avoids the climb and the bad junction is usually faster in practice and much less tiring.