Thanks for the mention careergirl 
I'm another keen runner who gave up in early pregnancy (nasty bleed) and adapted my regime to one which with the benefit of hindsight is a lot better for my health, appearance and fitness. It was a personal trainer who pointed out to me that endless running was doing nothing for me other than wreck my joints and burn calories as I find it easy, so I needed to stretch myself in other areas.
During pregnancy I worked on power walking (hilly terrain), squats, lunges, light weights, and worked out weekly with a personal trainer who devised a routine to strengthen and tone back, legs, arms, core. I also swam daily in an outdoor pool (usually 2km at moderate pace) until the weather meant I had to switch to more walking. I was walking approx 5 miles per day until 37 weeks, at which point SPD kicked in. I think next time round I will walk on flatter routes in later pregnancy as I think the steep hills overstrained my pelvis.
My advice is forget you are pregnant and keep up the exercise, just adjust your activities and lower the intensity. Getting hot and sweaty isn't advised, remember you are aiming for a maintenance regime. Also get advice from a qualified instructor as to what is/isn't ok. You need to consider things like protecting your stomach muscles from splitting, possible faintness in the second trimester when your blood volume ramps up, and avoiding lying on your back.
It is definitely worth keeping up the effort as there is evidence that exercise improves blood flow to the baby and gives them the best possible start. I certainly gave birth to a tough healthy little mite who was much bigger than expected given my skinny build. I also found labour much easier than I expected, and walked out of the labour room approx the same size as I was pre pregnancy. 5 weeks later I am back in a size 8, and I haven't had to lose a gram. I didn't curtail what I ate in pregnancy either - I ate loads, and was careful to get enough protein, fat and carbs. All I cared about was growing a healthy baby.
Enjoy your pregnancy.