I think the sales person's concern that your back will hurt- at least compared to an "all purpose" bike- is not unwarranted. When riding for more than like 30 minutes, my back muscles start to tighten up so I stop and stretch, refill my water bottles or whatever and then get back on the bike and repeat. Lame, yes, but that's how I do it and when I get back on the bike I'm having fun again.

I think that such discomfort could be discouraging to some people but a 26 yr old guy who has been athletically active for much of his life is probably just the kind of person who overcome muscular discomfort with conditioning or would just work with it as I do.

I don't think you should get an "all purpose" bike if what you want is to road cycle. There are many of us who started out with such bikes only to outgrow them within a few months and if you get one, you'll do the same and you'll take a loss when you resell it to get the road bike you wanted in the first place. Get a "sport" road bike, i.e. an entry level bike which is designed to spread you out a little but not very much. The emphasis is a compromise between comfort and performance. This is in contrast to an all-purpose bike which will sit you too upright and will compromise both aerodynamics and pedaling power (being more bent at the hip actually gives your glutes more leverage for the downstroke of the pedaling cycle) or a high-performance/racing bike which will spread you out quite a bit, place bigger demands on your lower back muscles and perhaps would indeed make you uncomfortable enough to discourage you from riding a road bike at all.