Hi, Kims! I don't know that much about allergy-related illnesses, but if you don't have the energy even for a 30 minute walk that could be a symptom of something more severe. I'll have fall back on that old mantra: check with your doctor about when aerobic exercise is contraindicated during a bout with your allergy(ies) and related illnesses.
I'm not sure what your overall program looks like, but one thing I learned over time that strength training boosts your energy level, for daily activities AND aerobic work, like you wouldn't believe. When I drop my norman lifting sessions even for one session, I feel sluggish, apathetic and tired; when I kick it back into gear I literally feel myself coming back to life.
I've been very peevish the past week or so because I weirded out my left foot, and that caused me to have to skip my normal Sunday cardio-strength monster workout, caused my to have to skip my normal Monday cardio-leg-circuit workout, and caused me to have to retool everything leg-oriented for the rest of the week. Today I was able to get a bit back into the swing of things, and I'll tell ya it felt great to get back to heavy weights.
Also - if you're not doing so already, consider bringing in aquatic training (swimming, water aerobics, aqua jogging) into your ongoing program. That can really keep you feeling limber and refreshed.
A-JOck