Is alder wood too hard to whittle?
No, it's not too hard.

For the beginner, Basswood and Butternut woods are the easiest to carve.  Don't be confused about the hardwood and softwood definitions as they aren't literal interpretations of the wood's hardness.

One source for carving woods is Heinecke Wood Products - https://www.heineckewood.com/

The Sculpture Studio has a list of some of the woods commonly used:

http://www.thesculpturestudio.com/typesOfWood.html

Metal is harder than all of the woods, but some wood is harder to cut than others due to the density, grain, and water content.

Sharpening knives

There are a lot of videos about sharpening knives out there.

Bottom line, start coarse and graduate to finer and finer grits.  At the end, stropping is the finest of the grits.

To strop, mount a piece of leather to a small board.  Rub a buffing compound into the leather and then pull the blade across it, backward so as to be actually honing the blade.  Besides the sharpening procedure, one of the keys to sharpening lies in the type of metal used to make the knife.

Cheap metal blades do not hold their edges very long and must be continually resharpened.

Good metal blades will require less sharpening and don't require such time consuming maintenance.  So choose knives carefully.

When stropping, the surface of the blade is being polished repeatedly.  This decreases the drag of the metal across the surface of the wood.

Up close, knife edges are really uneven and full of gaps and voids.  The goal in sharpening is to minimize them and create a smooth, even finish.

Longevity of the wood

Longevity of the wood depends on the environment it lives in.  Outdoor pieces will weather and be exposed to harsher, more extreme fluctuations which will cause cracking and checking.  Outdoor pieces should be coated with a UV and moisture resistant finish specifically designed for the exposure.

Safety

Do not overlook the need for a thumb guard and carving glove.  These items are truly essential.  A properly sharpened carving knife will cut, gouge, slice, poke, stab, impale, and hurt the carver.  Think about a leather apron which may prevent a cut to the legs and chest while carving in the lap.  The femoral artery is exposed on the inner thigh area and is easily cut without some form of protection.

Further reading

Great references for carving include
 Gene Messer - https://www.youtube.com/user/whittler0507
 Doug Outside (channel) - https://www.youtube.com/channel/UC-eOXKXJ2GQ1gewivwNxYKQ
 Kevin Coates - https://www.youtube.com/channel/UCHIwp6WJZdGvySIdpmxq5Yw

Happy Carving !