Over weight

Hi, I've just found out I'm pregnant, just 4 weeks. We have been trying for 5 months and I've been trying to lose weight up until then. I lost a stone but needed another off really. Now I'm a bit annoyed and disappointed that I didn't managed to get healthy and lean before we conceived. Is it too late to start doing a new exercise routine?  
Also I put On weight in the first place because I had a back problem and operation and couldn't exercise for 4 years( I was a personal trainer!!) now I still have back pain, but can push through it. 
any suggestions?