i do think hiring a rug doctor is worth every penny, and i am difficult to please as i detest wasting money.
Definitely use their products and spray the worst bits with the traffic lane spray or whatever it's called. Get a big bottle of the shampoo as that will last a while but don't be surprised if you use a whole bottle of spray.
The machines are heavy and quite large so you need to ensure your boot/back of car is big enough. When i hired them from Homebase they would always send someone out to the car park who would put it in the boot.
Also be prepared to move all your furniture and find hidden treasures lurking beneath the sofa! You also need to hoover first to get rid of loose dirt and grime, just makes it more efficient.
i always do the worst bit first so that when i have finished the rest of the carpets i can go back over that bit if necessary.
You do need a lot of hot water, and it is very hot work but satisfying. Carpets should dry quickly this time of year due to heating being on.
I used to do it every three months or so and inbetween just tried to keep the worst at bay without getting upset and annoyed as it gradually changed colour'.
It's not cheap but maybe you could think about going halves with a friend on cost if you rent it for 24 hours and both do your carpets? I have done that in the past.