If the inside of your scope fogs up you have lost the gas seal and its time to get a new scope.
I loaned a rifle to my uncle one year and it fogged on him and alas I had taken the sights off the rifle the previous season.. silly me.
My new rifle doesn't even HAVE sights, just the scope so I usually take a back-up rifle with me in then woods, behind the seat of the truck, etc. I ended up bringing the sights to camp, rezeroing the rifle's iron sights in 5 shots at 75 yards on a pie-plate.
I've also found that using scope covers in the field is a waste.. they can fog up, get glare, etc.. best to use those to keep dust/road dirt etc off the lenses while the rifle is in transport in your rifle rack or in your safe.
the BEST thing to have is a back-up rifle of some sort.
I've been known to switch off which rifle I carry depending on terrain/weather situation I'm going into. The only danger in doing this is ending up someplace you didn't intend with the wrong gun for the terrain because you are chasing elk.
My main rifle: savage model 116 fsak 30-06 with a tasco scope and burris mounts (burris mounts won't shoot loose when properly loctited), no sights. Shoots half inch groups all day.
Back-up gun is a 12 guage remington 870 with a deer barrel and a rifled tube the sights are zeroed at 50 yards and I can shoot a two inch group with it at that range.
There is also usually a 30-30 in camp and my brother ended up carrying THAT when he took a header off a rock face and whacked his rifle/scope/stock in an attempt to stay upright. The good news is he wasn't hurt.. the bad news is his old savage rifle got marred up pretty good.. scratches in the steel, gouge in the stock and a BIG ding on the scope.. rather than attempt to re-zero in the field and shoot up the woods doing so.. we just assumed the zero was off and took out a back-up gun.
I kinda wish now that I had bought a rifle with iron sights "just in case" but my back-up gun is never too far away in the truck so I don't worry too much.
Hope it helps,