"Well as I'm interviewing here I would obviously see taking this position as a step foreword for all the reasons we've discussed. Naturally I don't see this position as the job I'll work until I retire, and obviously I'll be looking for at least some mild expansion of responsibilities in the future. Whether that happens here or somewhere else in 5 years remains to be seen, but staying here is certainly an option depending on how things are going after 2-3 years. On that note, exactly how are company name's technical and managerial tracks structured..."
In that paragraph I've been honest, I've let them know that I don't intend to do menial labor forever and have aspirations, the minimum amount of time-loyalty they can expect from me, an assurance that I won't job-hop, and I've shown further interest in their company as well as the initiative to ask questions of my own if I haven't done so earlier.
Edit: Reading it here may sound like I want to be a manager in 3 years (which is crazy for a junior dev), but that's where manner and vocal tone come in.