Hard? Sure, leveling up makes your enemies tougher! Why not? But if you use a bit of common sense in improving your character's attributes, and selecting major skills, it's a piece of cake!
If you choose damn-fool things as your major skills - like alchemy, speechcraft, athletics, mercantile, illusion, etc, then your enemies will wipe the floor with you. In fact you may not make it past level 2.
You should choose major skills that will build up your strength and endurance, regardless of whether you are a fighter, a mage or a stealth class. Useful skills for STR and END are: any weapon, armourer. The rest are up to class type.
It is very helpful, too, to have decent armour. Personally, as early as possible I do the Main Quest so as to get sigil stones to enchant clothing with protective enchantments such as Fire Shield, Frost Shield, Shock Shield, and plain Shield. Use a shirt and pants and shoes. You can't get gloves, so you are stuck with gauntlets, but you can use an enchanted hood (I always enchant hoods with Nighteye) instead of a helmet. And rings and necklets are handy, too. And so are enchanted weapons.
Oblivion is a pretty easy game when you know how.