Hi,
Your classical history of pain starting from buttocks going down to calves and ankles making it difficult to walk long distances or walking fast indicates towards a condition called 
Sciatica. 
- Pain pattern is classical.
- Please consult an 
Orthopaedic Surgeon or Neurosurgeon for clinical evaluation.
- MRI of the lower spine will confirm or rule out the diagnosis.
- If the 
disc protrusion is too much you may need surgical correction.
If the prolapse of discs is not much then I would advise you the following:
- Consult a Physiotherapist for proper muscle training and exercises to improve.
- Take some anti-inflammatory with painkiller properties.
- Do not bend too much.
- Do not left heavy weights and avoid heavy physical work.
As for IBS the best way is the get 
CT scan of abdomen and 
colonoscopy done.
Tests of stool for three days and relevant tests of blood and urine. 
These may find the primary cause and thus a specific treatment is possible.
If all these investigations are normal then only one can label the problems as IBS that is 
irritable bowel syndrome and control of stress and 
anxiety play a big role in relieving your problems.
Take care. Hope I have answered your question. Let me know if I can assist you further.
Regards,
Dr. T. Chandrakant, ENT & General Surgeon