I've put 4 ET titles on my dogs over the years. Easiest letters ever to get!
My dogs are all kelpies and had a very high level of fitness to start with, so my special ET training was pretty minimal. My dogs were regularly accompanying me on 20 km rides out with the horse and I was a regular bike rider with them, travelling up to 40-50 km in a day. Needless to say, they could have gone around twice!
I will second the advice given to gradually build up your dog's fitness over a good 6-8 weeks, especially if the dog's normal activity is limited to a spin around the park. Also, it is good idea to run them on different surfaces - our old ET track here in Perth was limestone which could be problematic for dogs used to running on grass.
The rate at which the dogs are run during the test is 10 km/h, which is actually quite slow if you are on a bike. My bike had a speedometer thing attached which was quite handy in helping to regulate the pace. Don't speed up and then slow down as this will make the dog tired. Also make sure your dog is comfortable trotting along at the 10 km/h pace on lead, nothing like pulling a bike and handler along for 20 kms to really wear out a dog.
Good luck with the ET. Oh and yeah, the obedience test is easy peasy so take advantage of the on-lead option! I did my first ET back in the days when the obedience test was off-lead and included a retrieve over the jump. You can imagine how disappointing it was to pass the endurance part and then have the dog run around the jump at the end!