C.T.
A. congratulations how exciting. I am sure that everyone has you worried saying you should see a Dr right a way and while ideal you will be just fine and so will the baby if you can't see someone right away. The most important thing is to eat well lots of protein 80-100g a day, lots of veggies and if you are a processed food eater try to reduce that. You want to take prenatal vitamin with 400 mg of folic acid to ward off neural tube defects. Try to find a vitamin that is only one pill and says it is easy on the stomach. If you get morning sickness you will be pleased to only have to take one, some of them are huge. Also you might consider taking omega3 fatty acids (DHA's) which help with neuro development. Try to find one derived from plants not fish as there is some concern about the mercury that can be from fish. You are often advised not to eat soft cheeses because of listeria and also shell fish too. Look for fish with a low risk of mercury.
Down the road there will be testing that you want to do. Over 35 you are labeled as advanced maternal age and do have an increased risk of conditions such as downs syndrome. These tets can have a fairly high false positive so if you get positive results don't panic.
As you say you are fit already you are fine to keep exercising. The main thing is not to get too hot. For that reason hot tubs and saunas are not advised.
You don't need extra calories until the 3rd trimester so no eating for two yet :) But you may get quite hungry and tired during the first trimester. Getting off the couch can seem like an effort.
Remember many women don't realize they are pregnant until they are 3 months and everything is fine so get to a Dr when you can and just use common sense to look after yourself. Pregnancy is not an illness it is a state of being. Don't worry and enjoy the ride.
Good luck
C.