That's not necessarily true; travel by sail is generally much faster than travel over land. It was actually faster to take a ship from Dover to Capetown than it was to take a stagecoach from London to Edinburgh, for example, and the English channel is a much more formidable military obstacle than commercial and cultural barrier. If maps were measured by time, like how far one could travel in a day under muscle or sail power, well watered areas, like the vast Yangtze drainage network, the Nile river, and the Mediterranean would be relatively small, while Afghanistan, Southeast Asia, the Appalachian mountains would be immense.
Having England on the Spanish side not only lends their strength to the fight with the rebels, but nullifies or at least diminishes some of Spain's disadvantages. Instead of a major and minor power partnered against Spain, you have the world's superpower aligned with a great power against a tiny emergent confederation. Dutch and English attacks on commerce from the new world would be heavily curtailed, which gives Spain a greater edge over the Dutch, and with each advantage towards the Spanish, their strength outstrips the sum of their parts; more powerful forces are better able to inflict losses on lesser ones, which makes them comparatively even more powerful, thus compounding their ability to defeat their enemies.
If English naval forces in the North Sea could counter the Sea Beggars or deny them refuge in English ports, for example, the Dutch Revolt would have had a much harder time starting out, and the relatively greater tax revenue from the Netherlands could help the King avoid defaulting, thus allowing him to borrow more money at lower interest rates, letting him finance greater military force. If the tercios were paid properly, they might not have mutinied, devastating the country and alienating the people of the Netherlands. This is of course highly theoretical, but even with English support, the Eighty Years War was a close run thing, and Spain+England is dramatically less balanced than Spain vs Netherlands+England.