I don't know if I can recommend an exact LED to use (I'm constantly looking for better ones myself), but maybe I can point you in somewhat of the right direction.
Essentially, you want three things out of a bike light:
- Be seen by others.
- See what's right in front of you.
- See what is far away.
To start, your source voltage of 3.7 limits you to using 1 LED with current limiting resistor per parallel string. This isn't a bad thing, it just means you have to use more resistors as opposed to putting a few LEDs in series for double or triple that source voltage. The good news is that most lower-powered white LEDs have a forward voltage of 3 - 4V. I have to recommend that you DO use a resistor in series with each LED, even if the rated LED voltage is 3.7V. The LED current will fluctuate as it heats up and the LED forward voltage changes which can damage it or the battery. The resistor is there to make sure that current is limited.
You mentioned using 10mm LEDs rated for 25000mcd each @ 20mA. I'm guessing that the viewing angle on these is pretty low, such as 15 degrees. These types of LEDs are good for distance, but they don't work so well to light up things right in front of you. Here are some LEDs like that: 100 x 10mm 120000mcd @ 20mA, 12 degrees viewing angle - $40 (0.40 each). There are many LEDs for sale like this online.
If you are looking to use fewer LEDs that are higher power, something like this has a decent luminous intensity as well as a wide viewing angle: 62000mcd, 125 degrees - $12.95. The downside is that it will get very hot (needs the heat sink) and use quite a bit more current than your standard LEDs. This may be an even better choice: 300 lumens, 125 degrees - $6.95, but again, it will drain your battery a lot faster. However, pairing this type of LED with a focusing lens will reduce the viewing angle, but greatly increase the intensity. With the right lens, you can get everything you want out of one LED.
Consider the difference between a focused and unfocused flashlight: focusing the light beam increases the brightness, but less of the room is lit up; unfocusing the light reduces the brightness, but more of the room is lit up. To get the best light using the least amount of power, I would suggest you use a grouping of LEDs with a very high luminous intensity (mcd) paired with a group of LEDs with a high viewing angle (degrees). The easier route is to pair a high power LED with an appropriate lens, but this might not be ideal for you since you have such a small battery capacity. Either way will ensure you can see things far away, and the path in front of you is well lit.
Here is an online calculator which can estimate the output in lumens from the candella and viewing angle attributes: http://led.linear1.org/lumen.wiz
If you are trying to figure out your battery life, it can be estimated by dividing the amp-hour rating by the total current draw. This leaves you with the total hours of operation. Although, it isn't entirely accurate, and will vary based on the age and quality of the battery as well as the actual amount of current being drawn - some batteries do better with high discharge, some not so much.