The druid can't cast spells while in bird form flying or not unless they have the feat Natural Spell. If they do have the feat natural spell then there are a couple of things to consider.
What is the casting time of the spell in question?
The fly skill specifies
You generally need only make a Fly check when you are attempting a complex maneuver. Without making a check, a flying creature can remain flying at the end of its turn so long as it moves a distance greater than half its speed.
So, if the spell only takes 1 action. The druid could spend a move action to move a distance greater then half their speed and then cast a spell. This does not require a fly check or any other special considerations.
If the druid wishes to cast a spell that takes more then 1 action or they don't want to move up to half their speed. They can opt to hover. In order to hover they would need to make a fly check DC 15. This fly check is modified by the bird's maneuverability. This can range from a -8 to a +8 on the check. Also, the size of the creature also will apply a bonus or penalty. All of these can be found under the fly skill. Alternatively, the druid can take the feat hover in order to avoid having to make the check at all. Most creatures that have perfect maneuverability typically don't need the feat since the fly check to hover is trivial to them (eg. a medium air elemental gets +17 to fly meaning they make the dc 15 even on a 1).
In any case, since a fly check takes no action if the druid successfully hovers they do not have to spend a move action to keep from falling out of the air. This means they could cast as a full round action if needed.
As for flying long distances. I would just use the normal overland movement rules and use a terrain that is appropriate based on the weather conditions and prevailing winds. A favorable tail wind could be treated as a highway and a headwind could be treated as trackless. It would also depend on the type of flier. Most clumsy fliers are incapable of sustained flight.
As long as you have the feat Natural Spell a druid can cast while flying. This is really no different than casting a spell while walking. As LordKalas pointed out you only need to make a fly check while making a complicated maneuver. This is really no different than using running and acrobatics to tumble over things. If how you are moving (running or flying) requires anything more than a swift or free action it will impact the type of spells you can cast. If the movement takes a move action you can cast a spell that takes a standard action. If the movement does not take a move action then you can cast a full round spell.
The fact that the action you want to do is casting a spell does not make a difference. What is important is the type of actions you can perform. Generally if the flying creature would be able to make an attack action they can cast a standard action spell. If the flying creature can make a full attack action they can cast a full round spell. The only real difference is if the creature has a feat that specifically allows it to make a certain type of attack when flying than it may not be able to use that type of action to cast the spell. In this case the attack is being done because the feat allows it, not because they have that type of action available