With all the hours I put in FO3, I never "beat" the main storyline.
Its remarkably quick if you know how to do it.
When exiting Vault 101 rush straight to your dads vault.
Enter simulation, use special code to finish it instantly.
Let dad return to Rivet City on his own. Rest in a bed at Megaton to have him run there faster.
Head to Little Lamplight for GECK.
Use Child At Heart for instant access.
Go to vault, hug and cuddle Mutant for easy grabbing of GECK.
........
Race thru Enclave hideout to President. Speech to convince him to destroy Enclave.
GECK to dad.
.........
Run to Brotherhood.
Do Brotherhood stuff.
Final Assault (hang back and let robot do the work)
Take heavy companion into Jefferson Memorial (probably the Mutant again)
Watch final scene.
If you dont screw around it can probably be done in two or three game days.
And the final battles arent even hard, you just watch the mayhem then murder the final boss quickly with a minigun or gatling laser. With plenty of stims you might not even need heavy armor.
PROTIP: The Broken Steel DLC is an extension to the main quest, and its MUCH harder.