I was a lone developer for 4 years at my previous job. At first I was really enjoying the flexibility in making my own decisions and the speed with which I could do things without having to wait for group approval. But after a year or so I started to worry about the impact my developing on my lone island would have on my career. This is what I did to address my worries:
To counter the lack of other software dev input I started following tons of blogs on software development (although at the time they were more essays as opposed to quick blog format of today). I also bought books on software development methodologies and included their advice in my work to get a feel for their effectiveness.
To stay fresh and not isolate myself I kept trying to push new technologies (languages, frameworks, etc) into my work projects. That kept me both excited about what I was doing at work and also kept me on the cutting edge of tech. I was fortunate enough that I got flexibility to do that at work, provided I had a decent reason to use said new technologies.
I tried contributing to open source projects. Personally, I had a couple of bad experiences there and haven't tried it again since. But it's something to consider since obviously thousands of developers do it regularly and I'm sure they benefit from it.
In the end, I left my job to look for more interaction with other developers. I'm not sure if this was a result of the type of company I picked or not, but I found that technically I was ahead of the curve because of the constant diet of tech news/blogs I had fed myself on. But there definitely is great value to the questioning you get from other developers to your designs or implementation techniques. Not to mention experiencing first hand the politics of working in a group environment and the necessary techniques to navigate all that.
So if this is your first solo job, enjoy the benefits to the fullest and it will make you a better developer. But don't stick around for too long or you'll lose out on the other social aspects of development that are also just as important.