You could prune them and hope they do some more branching but I would wait until after they bloom this year or you'll cut off all you bloom. You need last season's growth for them to set buds. As soon as they're done blooming this season give them a good pruning and see if they send out more laterals for next year's bloom.

Another thing you can do with roses like these that want to climb or reach for the sun is to train the primary canes horizontally. You can either attach the canes to a surface behind them (like a trellis or pillar), or peg the canes to the ground in a fountain type shape. Paul Zimmerman has a nice video you can google about this. The reason they're blooming only on the end is that the rose puts its energies into the highest canes. When the main canes get horizontal, it will grow more laterals (side branches off the main canes, as Seil mentions), and those will flower a lot more along the length of the cane. You probably want to keep some primary canes unpruned with this method, if you want to encourage blooms this year, but you can always prune out underperforming canes or the oldest canes every year to increase blooms.

Cynthia

I'd prune them now before they leaf out, cut them back so they will branch out and get bushier with the summer's growth. They will still flower this year on the old wood that remains, just lower down. Since they only flower at the top, you aren't really losing any flowers by pruning them now, it's just more compact. And it'll be more self-supporting when it blooms. Long and leggy is likely to fall over in full bloom, especially when wet, or need staking.

The other thing you can do if you leave some long canes is to peg them horizontally so you get blooms along the stem. That will increase your blooms this year and you can cut them back after blooming to encourage a more bushy shape.