This is definitely a good idea. Cycling in Amsterdam is such an experience and can be a bit stressful in rush hours in the city center; hence, riding it through the country side can be more pleasant.
There are many companies renting bikes in Amsterdam. In general, the longer the opening hours and the more central the location, the more expensive they will be. However, you should get one for 24 hours for about 15€. Note you may be asked to leave a deposit (around 100€) and they will also suggest you to add some kind of insurance (around 5-10€).
Before leaving the shop, make sure of the following items:
- the bike is comfortable enough (you will be cycling many hours).
- lights and brakes work fine (be careful if they have pedal break, it is easy but needs some practice!).
- you know exactly what the conditions are.
- you get some map to help you.
- you wear some comfortable clothes, together with some spare t-shirt and some other "layers": another jacket, a scarf, etc. After all, you will be doing sport and it needs some changing clothes.
For a list of companies, you can check the I Amsterdam webpage (in English): Bike hire. You can also take a look at their section Cycling in Amsterdam.
For the route itself from Amsterdam to Marken, you can check the question How long does it take to bike to Marken Island from Amsterdam. It is around 60km, so you can go quietly, walk around the island and return within 5-6 hours without any problem.
You just need to take the free ferry from Centraal Station to the other side of the IJ. From there, follow the signs to Marken / Volendam.
I did myself that route once and I strongly recommend it. You can stop in different places such as Monnickendam or even Volendam if you have time. Then, the countryside is quite enjoyable, with many farms that even have some fruits and vegetables outside, so you can get some after leaving some coins in a money box.
Just make sure you have time enough to go back and forth. As you say, it can get dark at 16.30, so it is important that by time you are already in the outskirts of Amsterdam.
It is important to check the weather, whereas you can assume it will eventualy rain at a certain moment. It is the Netherlands! To protect from the rain you can go to a shop called HEMA (there are many throughout the city) and buy some protecting clothes. If you invest 20 euros you can get a full package (or a poncho) that is pretty much water proof and may be useful to you in the future. I do not recommend buying an umbrella, because riding the bike and holding it can be a bit tricky; also, rain normally comes with wind, so it can be useless.
Other interesting links regarding this route:
