What you did was cut off the buds that would have bloomed on the old/last years growth. Endless Summer should produce flowers on both new and old wood - but it may take longer for flower buds to develop on the new growth and your flowering may be later....it shouldn't mean no flowers at all as pruning now would have meant with most types macrophylla.

You should be fine as long there are "lateral buds" located on the sides of the stems. 'Endless Summer' will definitely flower using these buds. It's those HUGE blooms (9" +) that are generated by the terminal buds (the ones you cut off).

Don't worry...you should still have plenty of blooms come June.

As morz8 said, you also will get new-wood blooms as usual (think August).

From what I've seen from mine, some very large blooms come from the new thick 2ft+ stems that grow from the base each year (some as large as my head!), while the smaller ones (baseball/softball sized) come from the "lateral buds" off old stems. If the stems you cut were thick and had large swelled buds at the top, I'd say you lopped off some potentially big blooms, providing there was indeed a healthy undead bud up there to begin with. Lots of terminal buds seem to die off, especially in zone 5, so it's possible these were already dead.

Whatever the case, gradually pruning to your desired shape while taking off leggy stems will eventually pay off. And getting multiple ES's to grow at the same rate will take more equal light and soil conditions.