Was Gandalf wrong?
 

Just before the Company entered Moria, Boromir suggested that they might get to Gondor by taking the long way over the Isen and then reaching Gondor by the seaward route. But Gandalf contended that they "might take a year in such a journey". Surely that was a major over-estimate?

Gandalf's estimate was probably based on a worst-case scenario, involving periods of hiding, taking out-of-the-way paths to avoid troubles, finding food, and allowing for enemy actions from Sauron and Saruman.
Even if it wouldn't have taken such a long time as that, it was the longest road from which to choose, and time was pressing.

Also, Gandalf was factoring in the fact that Saruman should be avoided and Rohan can't be trusted. If the Fellowship went around their territories, they would have had to circle pretty much back to the Greyflood to get around Dunland and take a stroll down the beach. A loooong stroll down the beach. Assuming the beach was safe from raiders and other nasties.

So, as Inzil said, maybe it wouldn't have taken a year, but definitely a very long time.

If they wanted to get to Gondor unharmed, it would've taken at least half a year.

Knowing Gandalf, it was probably just an exaggeration, though.