The Stormcloaks only really work if they were content on not leaving their borders.
They have a few options.
1) they sail by sea around the Western Coast of Tamriel, which really limits the window time to start their invasion due to the deadly storms in the northern regions. Take a few weeks or months to get to where they're landing, so reinforcements/supplies will take a long time to get to the front lines and communication between the invasion forces and Skyrim will be poor, only being able to send messages every few weeks (Unless you had Battlemages to contact other Battlemages... Which they don't have). You're facing the powerful Aldmeri navy, who carry Battlemages on ships. Besides not having Battlemages, Nord Captains would most likely refuse to even transport Mages due to it being considered bad luck.
2) March through Hammerfell, going across a desert probably isn't the best idea for Nords. Be a strain on any army, eat up a lot of resources and it would be very slow going. Stopping at the water oases every day/only marching at night.
3) Go through Cyrodiil, the northern passes are a real bitch. They close up all the time, and you're facing issues with supplies like number one. Long distance to cover, harsh environments etc. Unless you're relying on the Empire to supply you, and that really defeats the purpose of going independent and being self sufficient.
"We're done fighting for a corrupt and dying Empire, so weak and oppressive... By the way could you supply us with food, clothes, wood & metal? Be a huge help... Skyrim belongs to the Nords, though."