Recent years have witnessed a growing interest in reconfigurable antenna systems. Travelling wave antennas (TWAs) and leaky wave antennas (LWAs) are representative examples of structures featuring a great level of flexibility (e.g., straightforward implementation of beam scanning), relatively simple geometrical structure, low profile, and low fabrication cost. Notwithstanding, the design process of TWAs/LWAs is a challenging endeavor because efficient handling of their electrical/field characteristics requires repetitive full-wave electromagnetic (EM) analyses, which is computationally expensive. In this paper, a novel approach to rapid optimization of LWA's input characteristics is proposed, based on structure decomposition and rendition of fast surrogate models of the antenna unit cells. The surrogates are combined into a single metamodel representing antenna input characteristics, which enables low-cost adjustment of the geometry parameters. The presented methodology is demonstrated through the design of several LWAs operating in the frequency bands of 8.2 GHz to 11.2 GHz, 6.2 GHz to 8.2 GHz, and 3.8 GHz to 4.7 GHz. Numerical results are validated through physical measurements of the fabricated array prototype.