I am just a recreational rider, I don't have to do interval training ..... do I?

You don't have to do interval training but if you want to be faster, or just more comfortable riding your normal pace, intervals will make you stronger on the bike.
What is an “Interval”?   In 'Interval' is any period of time where your pace is faster or harder than normal.   Intervals can range from 15 seconds to 30 minutes.  Obviously, you can't ride the same pace for 30 minutes that you can for 15 seconds. Therefore, the shorter the interval, the faster one typically rides.   Longer intervals (10 min and longer) are typically done at a time trial pace.
Why do Intervals?    By riding faster than normal for discrete periods of time, you force your body to work harder than you would for your normal cruising speed.  You can't do the whole ride at this pace, but by breaking them into discrete periods, or “intervals”, you can sustain a higher speed for that shorter period of time.    Now, you may be wondering why you need to go faster if you don't plan to ride any faster.  Let's say you want to do Ragbrai and you normally ride around 14 MPH and you are happy with that.   Why should you train any faster than that?   Good question!  There are reasons why you should train faster than you plan to ride.   First, by training at a faster speed, you force your body to adapt to a higher demand.  Your heart, lungs, blood and muscles all adapt and become more efficient.  This allows you to do two things better – you can ride your normal pace more easily because your body is more efficient and you aren't riding at as near your maximum.  If  you train at 16 MPH and ride at 14, 14 becomes easier (and therefore more enjoyable).     You now have greater aerobic capacity and you have some energy in reserve.  Again, if you normally are riding at 14 MPH, you aren't maxed out at 14.  You have something left if a hill or headwind gets in the way.  Or if your friend  takes off and tries to leave you behind.
How do you do an Interval?    Although there are a lot of sophisticated ways to do intervals using heart rate or power meters, all you really need to do is ride a little faster for a given period of time, recover and do it again one or more times.  One easy way to do them is just randomly.  Let's say you are riding through town to get to the bike trail.  Every time you come to a hill, step on the pedals and accelerate up the hill.  Spin easily on the level and downhills.   Or, when you come to a stop sign or stop light, accelerate out of the stop and hold it for 30 seconds.  If you like structure, you can head out to an open road (straight, flat, no stops work best) and pick up the pace a couple MPH for a given duration of time.  30 seconds or one minute are convenient intervals.  You can then take a rest period equal to the work period (30 seconds on, 30 seconds off) or come up with your own pattern.  Do as many as you can until you can't maintain that pace any longer.
When should you do Intervals?   Intervals are a hard workout  - that's why  they make you faster.  However, you should only do these once or twice a week at the most.  Always give yourself at least two days between interval sessions as you need to allow your body to recover for at least 48 hours.  For example, you might do them on a Tuesday and then again on Saturday.   A good time to do them is during one of your long rides.  If you are going out on a two hour ride, do a 5  minute interval every 30 minutes.   This will add some speed to your 'long slow distance' rides.
So give them a try, they aren't just for racers.   Imagine yourself riding a century ride this year and being able to comfortably ride your previous pace and have some reserve left when your buddy/spouse/arch rival tries to pull away.  Or better yet, imagine dropping them!
 
Maybe I'm finding this so the group will slow down so I wont get left behind :)
 
Dave