Going Potty!

Today was a big breakthrough in our Potty Training! Logan went to the potty 4 times today and did it pretty much on his own! I was kind of waiting to start potty training until the end of the summer and our traveling but this morning, Logan came into our room and was laying on our bed and he said, "I'm peeing! I need to go to the potty!" so we rushed in and he went! This has happened before. He has actually tried going to the potty off and on for almost a year, but he has never been consistent about it. From some advice I have received, I have just been kind of waiting for him to seem ready and I guess maybe he finally is. He did have a couple wet diapers in between the trips to the potty, but he did pretty good and told me most of the times he needed to go. He actually had one poopy one diaper too, but I will blame that one me and Darren. Darren was on the phone, so he sent him to find me and I was cooking, so I sent him to find Darren and he didn't make it in time. I have been giving him 5 M&Ms for going #1 and 10 for #2 and that seems to be working. I have also been bribing him with Lightning McQueen underwear, so we will be picking those up soon, and a pet fish! Maybe I am jumping the gun... We'll see how tomorrow goes.
As much as I hate changing and buying diapers, I am a little nervous to start the underwear! I don't want to clean up the messes and I am worried about all of the "I Have to go Potty" moments at inconvenient times like in the car, or in the middle of grocery shopping. Any advice on handling this?